Trip of French police a masterly stroke (4,2,5)
I believe the answer is:
tour de force
'a masterly stroke' is the definition.
'tour de force' can be an answer for 'stroke' (thesaurus). I'm not sure about the 'a masterly' bit.
'trip of french police' is the wordplay.
'trip' becomes 'tour' (a journey or expedition).
'of french' becomes 'de' ('of' in French).
'police' becomes 'force' (the force can refer to the police force).
'tour'+'de'+'force'='TOUR DE FORCE'
Can you help me to learn more?
(Other definitions for tour de force that I've seen before include "Outstanding feat" , "Highly skilled performance" , "Magnificent performance" , "Masterly or brilliant feat" , "Superb bit of work" .)