Gangster is more active around summer time (7)
I believe the answer is:
mobster
'gangster' is the definition.
('mobster' can be a synonym of 'gangster')
'more active around summer time' is the wordplay.
'active' is an anagram indicator (the letters are active/moving).
'around' means one lot of letters goes inside another.
'summer time' becomes 'bst' (British Summer Time).
'more' with letters rearranged gives 'moer'.
'moer' placed around 'bst' is 'MOBSTER'.
'is' is the link.
(Other definitions for mobster that I've seen before include "Mafioso" , "family man" , "Organised criminal" , "Member of criminal organisation" , "Gangster" .)