Change dates quickly to be loyal (9)
I believe the answer is:
steadfast
'loyal' is the definition.
(I've seen this in another clue)
'change dates quickly' is the wordplay.
'change' is an anagram indicator.
'quickly' becomes 'fast' (synonyms).
'dates' with letters rearranged gives 'stead'.
'stead'+'fast'='STEADFAST'
'to be' acts as a link.
(Other definitions for steadfast that I've seen before include "Sure" , "Dedicated" , "Constant and resolute" , "Resolute, unwavering" , "Fixed, unyielding" .)