In error a boy becomes a Muslim warrior (7)
I believe the answer is:
saladin
'a muslim warrior' is the definition.
(I've seen this before)
'in error a boy' is the wordplay.
'in' indicates putting letters inside.
'error' becomes 'sain' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a boy' becomes 'lad' (informal term for boy).
'sain' enclosing 'lad' is 'SALADIN'.
'becomes' acts as a link.
Can you help me to learn more?
(Other definitions for saladin that I've seen before include "Enemy of Richard I in the Crusades" , "Richard the Lionheart's Crusade opponent" , "He fought Richard" , "Richard's enemy" , "Sultan who fought the Crusaders" .)