Resolute, fixing dates quickly (9)
I believe the answer is:
steadfast
'resolute' is the definition.
(similar in meaning)
'fixing dates quickly' is the wordplay.
'fixing' indicates anagramming the letters.
'quickly' becomes 'fast' (synonyms).
'dates' with letters rearranged gives 'stead'.
'stead'+'fast'='STEADFAST'
(Other definitions for steadfast that I've seen before include "Firm and unwavering" , "Sure" , "Dedicated" , "stable" , "Resolutely firm" .)