Punk with green hair brushed up (7)
I believe the answer is:
ruffian
'punk' is the definition.
(thesaurus)
'green hair brushed up' is the wordplay.
'green' becomes 'naif' (similar in meaning).
'hair' becomes 'fur' (fur is a kind of hair).
'brushed up' says the letters should be written in reverse (in a down clue, letters go up).
'naif'+'fur'='naiffur'
'naiffur' written backwards gives 'RUFFIAN'.
'with' is the link.
(Other definitions for ruffian that I've seen before include "Yobbo" , "Aggressive, rude troublemaker" , "Rowdy" , "Lout" , "Violent criminal" .)