There is a screw right on the top of the trigger, that clamps it to the pivot pin. You need to remove the frame in order to access this screw. Sounds like yours has come loose. This is assuming that you have the stock trigger - some aftermarket triggers do not have this screw.
There is no clip. Not sure where you got that from, as I didn't mention a clip.
Right on the very top most part of the trigger, there should be a little grub screw. This tightens down on to the pivot pin. If the screw isn't tight, the trigger will move around and the pin will be loose.
Basically, the pivot pin passes through a horizontal hole near the top of the trigger. There is a threaded hole on the very top of the trigger at right angles to the hole for the pivot pin. You tighten the screw down on to the pin to lock it in place - it was the locking action I was referring to when I used the word 'clamp'. Poor choice of words on my part.