Stiff, unbending (5)
I believe the answer is:
rigid
I believe this clue is a double definition.
'stiff' is the first definition.
(I know that stiff can be written as rigid)
'unbending' is the second definition.
(I know that unbending can be written as rigid)
(Other definitions for rigid that I've seen before include "Stiff and inflexible" , "Stiff, unbending" , "Hard" , "completely" , "Frozen" .)