Your Sequence is a mismatch to your source footage. ie. different sizes.
Create a matching Sequence and you will be golden eg 1920 x 1080.....
Shooternz is right. Your clip and timeline sequence likely don't match. The easiest way to get your particular clip to match is by simply right clicking on the clip in the Project Bin and then selecting "New Sequence from Clip." If the rest of your footage is the same aspect ratio, then you can simply continue editing.