Firm dates up quickly (9)
I believe the answer is:
steadfast
'firm' is the definition.
(I know that firm can be written as steadfast)
'dates up quickly' is the wordplay.
'up' indicates an anagram (letters get chucked up).
'quickly' becomes 'fast' (synonyms).
'dates' is an anagram of 'stead'.
'stead'+'fast'='STEADFAST'
(Other definitions for steadfast that I've seen before include "Constant and resolute" , "Firm and dependable" , "Constant, not changing or yielding" , "Firm and unwavering" , "Resolute, unwavering" .)