Boy wonder, at last one becomes an outlaw (7)
I believe the answer is:
ladrone
'an outlaw' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I can't see?
'boy wonder at last one' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'at last' suggests the final letters.
The final letter of 'wonder' is 'r'.
'lad'+'r'+'one'='LADRONE'
'becomes' acts as a link.
Can you help me to learn more?
(Other definitions for ladrone that I've seen before include "robber" , "he'd nip there once" , "Old pirate" .)