Illegal use of dope when part of team, customary within group (7,7)
I believe the answer is:
insider trading
'illegal use of dope' is the definition.
(using insider information or 'dope' illegally)
'part of team customary within group' is the wordplay.
'part of team' becomes 'insider' (insider can mean someone in a team).
'customary' becomes 'trad' (trad can mean traditional).
'within' becomes 'in'.
'group' becomes 'g' (this might be a standard abbreviation which I don't know about).
'insider'+'trad'+'in'+'g'='INSIDER TRADING'
'when' is the link.
(Other definitions for insider trading that I've seen before include "Illegal stock dealing" , "white-collar crime" , "Dodgy dealing" , "Illegal stock exchange activity" .)