Witty remark, gem, pair in convulsions (7)
I believe the answer is:
epigram
'witty remark' is the definition.
(I've seen this before)
'gem pair in convulsions' is the wordplay.
'in convulsions' is an anagram indicator.
'gem'+'pair'='gempair'
'gempair' is an anagram of 'EPIGRAM'.
(Other definitions for epigram that I've seen before include "Quip, witty saying" , "witty comment" , "A clever witty saying" , "short satirical poem" , "Form of wit" .)