Unshakeable (7)
I believe the answer is:
adamant
'unshakeable' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for adamant that I've seen before include "unshakeable in purpose" , "Refusing to change one's mind" , "Unyielding, won't change mind" , "Decided" , "Definite and unyielding" .)