I'd be surprised if he has swallowed something so big (but I guess not impossible)
Have you checked the bedding to see if it's fallen off in there?
Maybe still attached to the cage & the rest of the nibbles fallen off?
Have a thorough search to try & find it.
If he's still eating, pooping & peeing they're all good signs
If you can't find the clip, If he's acting lethargic or hunched, probably facing a corner, hair sticking out/puffed up, acting unusually or stops eating,drinking, pooing, peeing then take him to a vets ASAP he'll probably need an xray to check he's not swallowed it!
Just a little whiff of gas may be necessary to keep him still enough to do an xray. But it should show up easily even if he's just popped in a cardboard shoe box or similar. It's rather large & metallic so the image should be really clear!
If it's a plastic clip it should still show up, but not as clear or white/dense in the image.
Can you get to an out of ours vet sooner rather than later? If it causes a blockage it could be serious.
Can you check his mouth/teeth to check it's not caught around it /them or caused any damage to either?
Good luck & fingers crossed you find it. Let us know how you get on xx
Ps... Even if you do find it & he's still lethargic keep a close eye on him & get him checked over for other possible illnesses. Xx