Once per crank revolution. Just when one crank is forward and usually when you are putting some power into the pedal. And you feel it more than you hear it?

Then I'd first suspect the pedal. Specifically the bearing adjustment. Either clean, lube and readjust the bearings. If it is a pedal that allows that. Or just simply get new pedals. If you have other pedals you can swap to that bike just to ascertain that the pedal is the issue, then that might be the better thing to try first. Especially if working on the pedal isn't something you have the tools or experience with.

However there are a lot of things it could be. And it'll help to know for sure if this is one click per pedal/crank revolution. Or multiple clicks.