Well, to be specific, in your case you want to use Fill Left--yeah, it seems backwards. Basically, the effect is saying, "Take audio from channel specifically named in the effect, and copy it into the other channel."
If you've already used the clip the Fill effects are probably the quickest way to achieve what you want. However, if you hadn't used the clip already, you could remap the stereo audio into a mono pair; you'd do this by selecting a clip in the bin, and going to Clip > Modify > Audio channels, and then using the options there to set up the audio as mono. When mono audio is used in a sequence with a stereo master, it's automatically routed to both output channels.
A more complex but less "permanent" approach would be to use submixes. You could pan the track to the left, and then send to a mono submix--this would of course mean that you had only the clips that you wanted to pan in that particular track.
There are a quite a few ways to go about this; it just depends where in the editing process you are, and how flexible you want editing to be for those clips going forward.
That modify audio channels is a very handy option. However I miss Final Cut Pro on moments like these. You could just turn of the stereo link.
I was already looking at them submixes before, but didn't understand how that worked.
Thanks for your help!
You could just turn of the stereo link.
Right, and that's because to FCP, ALL audio is mono--just paired or not paired. Premiere Pro is different because you can have clips and tracks with mono, stereo, or 5.1 audio, and you need to match them up. It's a little weird at first--I came from Avid which is similar to how FCP works--but you'll get used to it and actually start to appreciate the flexibility afforded by this approach.
If you want to replicate FCP's handling of stereo audio, however, you can set this as a program-wide preference so you should never have to think about it again. Go to Preferences > Audio, and set the Audio Channels default track format option from "Use File" to "Mono." Now, whenever you import a clip with polyphonic audio, it will be treated as though it has multiple mono tracks, e.g. a stereo clip will have 2 mono tracks instead of 1 stereo track. It's not changing anything with the source file; just how Premiere Pro interprets it.
Note that this will not force panning, however. If you put a dual mono clip in sequence with a stereo master track, both channels will be equally routed to both output channels (left and right). If you want, say, Channel 1 to go left and Channel 2 to go right, you'll have to pan those in the Audio Mixer on a track level. Just something to be aware of.