I will assume you do not mean the natural highlight on the pupil but instead, the equally natural highlight on the lower eyelid.
Its probably not specular and has some detail. Duplicate the layer, set the blend mode to multiply. Mask off the entire layer except the thin line that concerns you.
Clone is far too 'sledge-hammer to crack a nut'. What I do is sample the colour around the highlight, and build it up on a new layer with a fully soft brush set at 10% opacity. Keep on resampling the colour as you go. Go a wee bit further than you'd think right - so you can see the edit.
Then turn the layer into a Smart object, and add Gaussian blur. Being a smart object you can edit the blur if you go to far. The blur makes the edit melt into the surrounding area, but also thins it, which is why you need ed to go a wee bit too far.
If it is too thin, duplicate the layer.
1. Alt + click new layer
2. Choose mode as "overlay"
3. Check the check box beginning with "fill with ..." and click ok.
4. Choose a soft brush
5. Set flow to 2-5 %
6. Set opacity 10 - 20 %
7. Set your foreground color to black
8. Paint the area with very small strokes
You may try this method with different foreground colors (especially with the skin colors taken from the photo) and with different brush settings.
This is the best method as far as I know ... and you may google about it as "pixel based dodge and burn"
I am not being able to darken the birght spots enough - its like where I pass the brush everything goes darker, not only the bright white spots. So there remains a significant difference between these spots and the mid-tones around them. I don't know if I am doing something wrong.
any additional advice? the opacity of the brush?
Although this image is not a perfect candidate for this technique, if you use very small strokes in this image, I think you can get nice results.
There is also a technique as a shortcut to this one ... If you need to smooth larger areas, you may also use "inverted high pass" which is the non-labor intensive version of dodge & burn. Both of them are used in beauty retouching work for smoothing the contrasty skin areas without loosing the skin texture and pores. But, dodge & burn always comes first if the quality is the main concern.
If you do a lot of portrait works, please google about them ... sure you will find some nice training videos.
As is always the case in PS, there are a number of ways to accomplish the same task. Some work better than others, depending on the nature of the task. Here are two others:
• using the healing brush (not the spot healing brush), sample an area of good skin and brush over the offending area once. Then use fade filter and lower the opacity until the blend looks natural. The skill of this technique depends on the area you sample from and how large a brush / area you brush over. You only want to brush once, so you can use the fade filter command. An alternative, depending the version of PS, is to apply your healing on a separate layer and then lower the opacity of that layer. This is the same as using the fade filter command, but allows you to use multiple brush strokes.
• on a blank layer, using a small soft edge brush, low opacity - 10% is good, sample the desired good skin color near by the area you want to change. You may need to sample different areas as you retouch to get a good blend - transition. Then paint over the offending area, gradually building up the retouch until the blend is good.
In all cases a tablet is the best way to administer retouching.