Refusing to give in the end natural (8)
I believe the answer is:
stubborn
'refusing to give in' is the definition.
'the end natural' is the wordplay.
'the end' becomes 'stub' (I've seen this before).
'natural' becomes 'born' ('born' can be similar in meaning to 'natural').
'stub'+'born'='STUBBORN'
(Other definitions for stubborn that I've seen before include "intractable" , "Tenaciously unwilling to yield" , "Resistant to change" , "Obstinate, bullheaded" , "Difficult to move" .)