Posy of flowers, say, gone rotten (7)
I believe the answer is:
nosegay
'posy of flowers' is the definition.
(I've seen this before)
'say gone rotten' is the wordplay.
'rotten' indicates anagramming the letters.
'say'+'gone'='saygone'
'saygone' anagrammed gives 'NOSEGAY'.
(Other definitions for nosegay that I've seen before include "pick of the bunch" , "Old-fashioned posy" , "Fragrant posy" , "spray" , "Fragrant bunch of flowers" .)