Don't give in so much - be persistent (10)
I believe the answer is:
relentless
'persistent' is the definition.
(I know that persistent can be written as relentless)
'don't give in so much' is the wordplay.
I cannot really understand how this works, but
'give in' could be 'relent' (I've seen this before) and 'relent' is found within the answer.
The remaining letters 'less' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'be' acts as a link.
Can you help me to learn more?
(Other definitions for relentless that I've seen before include "Oppressively constant; inflexible" , "Harsh, inflexible" , "sustained" , "Inexorable" , "Dogged" .)