Determined, crafty little supporter within (6)
I believe the answer is:
steely
'determined' is the definition.
(I've seen this before)
'crafty little supporter within' is the wordplay.
'crafty' becomes 'sly' (synonyms).
'little supporter' becomes 'tee' (a tee is used to support a golf ball).
'within' indicates putting letters inside.
'sly' enclosing 'tee' is 'STEELY'.
(Other definitions for steely that I've seen before include "Obdurate" , "Hard and metallic" , "Gimlet-eyed" , "Set off" , "Determined; severe" .)