Say, collar a thug (5)
I believe the answer is:
rough
'a thug' is the definition.
(both can mean a violent person)
'say collar' is the wordplay.
'say' indicates a 'sounds like' (homophone) clue.
'collar' becomes 'ruff' (ruff is a kind of collar).
'ruff' sounds like 'ROUGH'.
(Other definitions for rough that I've seen before include "Sketchy" , "Not gentle or restrained" , "Rowdy - part of golf course" , "Not smooth, even or regular" , "Nasty" .)