Collar the boy - he's a hoodlum (7)
I believe the answer is:
ruffian
'a hoodlum' is the definition.
(I've seen this before)
'collar the boy' is the wordplay.
'collar' becomes 'ruff' (ruff is a kind of collar).
'the boy' becomes 'ian' (man's name).
'ruff'+'ian'='RUFFIAN'
'he's' is the link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ruffian that I've seen before include "Scoundrel" , "Rude fellow" , "Lout" , "Rowdy" , "Thug" .)