Stunning success allowed in verse (7)
I believe the answer is:
couplet
'verse' is the definition.
(I've seen this before)
'stunning success allowed' is the wordplay.
'stunning success' becomes 'coup' (I've seen this in another clue).
'allowed' becomes 'let' (to let is to allow or permit).
'coup'+'let'='COUPLET'
'in' acts as a link.
(Other definitions for couplet that I've seen before include "Pair of verse lines" , "piece of poetry" , "Two lines of verse, often rhyming" , "Two successive lines of verse, usually rhyming" , "Pair of lines in poetry" .)