Insult that's set out to stifle panache (4,2,3,4)
I believe the answer is:
slap in the face
'insult' is the definition.
(I've seen this before)
'set out to stifle panache' is the wordplay.
'set out' is an anagram indicator.
'stifle'+'panache'='stiflepanache'
'stiflepanache' anagrammed gives 'SLAP IN THE FACE'.
'that's' is the link.
(Other definitions for slap in the face that I've seen before include "the result of stupidly eating pancake?" , "Insult" , "Unexpected rejection" , "Unexpected affront" .)