Authorise drug arrest by the French (6)
I believe the answer is:
enable
'authorise' is the definition.
(thesaurus)
'drug arrest by the french' is the wordplay.
'drug' becomes 'e' (e is another name for ecstasy).
'arrest' becomes 'nab' ('nab' can be a synonym of 'arrest').
'by' says to put letters next to each other.
'the french' becomes 'le'.
'e'+'nab'+'le'='ENABLE'
(Other definitions for enable that I've seen before include "Give power or capacity to" , "Render capable" , "Capacitate" , "Permit or facilitate" , "Make it possible for someone to do something" .)