Wise guy putting cover on chimney (7)
I believe the answer is:
hoodlum
'wise guy' is the definition.
(wise guy can mean a gangster)
'cover on chimney' is the wordplay.
'cover' becomes 'hood' (hooding is a kind of covering).
'on' means one lot of letters go next to another.
'chimney' becomes 'lum' (I've seen this in other clues).
'hood'+'lum'='HOODLUM'
'putting' acts as a link.
(Other definitions for hoodlum that I've seen before include "Aggressive young criminal" , "Mobster" , "One breaks the law" , "Lawless youth" , "Young thug or gangster" .)