Firm revises dates quickly (9)
I believe the answer is:
steadfast
'firm' is the definition.
(I know that firm can be written as steadfast)
'revises dates quickly' is the wordplay.
'revises' indicates anagramming the letters.
'quickly' becomes 'fast' (synonyms).
'dates' anagrammed gives 'stead'.
'stead'+'fast'='STEADFAST'
(Other definitions for steadfast that I've seen before include "Strong" , "Resolutely unwavering" , "Firm and dependable" , "Firm and unwavering" , "stable" .)