Now I'm thinking it has to be either PESTO or PASTA but I really can't see how it works with the clue. I thought rebelliously was an anagram indicator but the P and S are definitely correct...
I think it is:
PASTA = SAP< + TA
A "nelly" is a silly person or a SAP and rebelliously is a reversal indicator in a down clue; think of a rebellion as an uprising. (It could also be an anagram indicator, but that would mean an indirect anagram here, which is generally frowned upon.) "Cheers!" means thanks or just TA.