I persist, for determination in the end works (5,6)
I believe the answer is:
first person
'i' is the definition.
'persist for determination in the end works' is the wordplay.
'in the end' suggests the final letters.
'works' indicates anagramming the letters (letters worked into a new form).
The final letter of 'determination' is 'n'.
'persist'+'for'+'n'='persistforn'
'persistforn' with letters rearranged gives 'FIRST PERSON'.
(Other definitions for first person that I've seen before include "Rebecca's written in it" , "Adam" , "I or we (grammar)" , "royal "we", say" , "was a faller at the outset" .)