Two lines of verse (7)
I believe the answer is:
'two lines of verse' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for couplet that I've seen before include "Pair of lines in poetry" , "Two lines of verse, often rhyming" , "Pair of verse lines" , "Two successive lines of verse, usually rhyming" , "piece of poetry" .)