In error a young man became Muslim warrior (7)
I believe the answer is:
saladin
'muslim warrior' is the definition.
(I've seen this before)
'in error a young man' is the wordplay.
'in' means one lot of letters goes inside another.
'error' becomes 'sin' (a sin is a moral error).
'young man' becomes 'lad' (lad can mean a boy or young man).
'a'+'lad'='alad'
'sin' enclosing 'alad' is 'SALADIN'.
'became' is the link.
(Other definitions for saladin that I've seen before include "Richard's opponent" , "Richard the Lionheart's Crusade opponent" , "Old sultan" , "Enemy of Richard I in the Crusades" , "Richard's enemy" .)