Enclosed rhyme scheme (4)
I believe the answer is:
abba
'enclosed rhyme scheme' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for abba that I've seen before include "Old group" , "rhymes in Petrarchan sonnet" , "6 24 113 [SCAN-DIN-AVIAN] band" , "Eurovision winners" , "Pater's" .)