Adamant (8)
I believe the answer is:
resolute
'adamant' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for resolute that I've seen before include "Determined or decided" , "'Determined, dead set on (8)'" , "Determined, unwavering" , "in a determined manner" , "'With fixed purpose, steadfast (8)'" .)