Tight on dates perhaps, but unwavering (9)
I believe the answer is:
steadfast
'unwavering' is the definition.
('steadfast' can be a synonym of 'unwavering')
'tight on dates perhaps' is the wordplay.
'tight' becomes 'fast' (both can mean secure).
'on' says to put letters next to each other.
'perhaps' indicates an anagram.
'dates' is an anagram of 'stead'.
'fast' put after 'stead' is 'STEADFAST'.
'but' acts as a link.
(Other definitions for steadfast that I've seen before include "stable" , "Resolute, unwavering" , "Constant and resolute" , "Constant, not changing or yielding" , "Dependable, unwavering" .)