Unbending; determined (6)
I believe the answer is:
steely
This clue is a double definition.
'unbending' is the first definition.
(I've seen this in another clue)
'determined' is the second definition.
(I've seen this before)
(Other definitions for steely that I've seen before include "Strong" , "Determined; severe" , "As hard, or grey, as metal" , "With (cold) resolve" , "Ruthless" .)