Insistent (7)
I believe the answer is:
adamant
'insistent' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for adamant that I've seen before include "Unswerving" , "Unpersuadable" , "Refusing to change one's mind" , "Resolute; diamond (arch.)" , "Unshakable (in an opinion)" .)