Finish off second issue for 15 (3,2)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

MO (short for moment = second = brief time) + PUP (for example a seal pup, the offspring or “issue” of 15 down ARCTIC SEAL). Mop up = to deal with the last few bits of a task, after the majority has been done.