I’ve struggled in pain to succeed (7)
I believe the answer is:
achieve
'succeed' is the definition.
(achieving is a kind of succeeding)
'i've struggled in pain' is the wordplay.
'struggled' indicates anagramming the letters (I've seen 'struggling' mean this).
'in' means one lot of letters goes inside another.
'pain' becomes 'ache' (ache is a kind of pain).
'ive' anagrammed gives 'iev'.
'iev' placed into 'ache' is 'ACHIEVE'.
'to' is the link.
(Other definitions for achieve that I've seen before include "wrap up" , "Successfully conclude" , "win" , "Successfully bring off" , "Attain successfully" .)