Resolved to alter the dates quickly (9)
I believe the answer is:
steadfast
'resolved' is the definition.
(thesaurus)
'alter the dates quickly' is the wordplay.
'alter' indicates anagramming the letters.
'quickly' becomes 'fast' (synonyms).
'dates' anagrammed gives 'stead'.
'stead'+'fast'='STEADFAST'
'to' acts as a link.
(Other definitions for steadfast that I've seen before include "Unfaltering" , "Strong" , "Constant, not changing or yielding" , "Firm and unwavering" , "Resolutely firm" .)