Firm dates rotting quickly (9)
I believe the answer is:
steadfast
'firm' is the definition.
(I've seen this before)
'dates rotting quickly' is the wordplay.
'rotting' is an anagram indicator.
'quickly' becomes 'fast' ('fast' can be a synonym of 'quickly').
'dates' is an anagram of 'stead'.
'stead'+'fast'='STEADFAST'
(Other definitions for steadfast that I've seen before include "Constant, not changing or yielding" , "stable" , "Sure" , "Constant and resolute" , "Firm and dependable" .)