Policeman’s good aim with weapons, mostly (8)
I believe the answer is:
gendarme
'policeman's' is the definition.
(gendarme is a kind of policeman)
'good aim with weapons mostly' is the wordplay.
'good' becomes 'g' (abbreviation).
'aim' becomes 'end' (both can mean a goal or purpose).
'with weapons' becomes 'armed' (I've seen this in another clue).
'mostly' means to remove the last letter.
'armed' with its final letter taken off is 'arme'.
'g'+'end'+'arme'='GENDARME'
(Other definitions for gendarme that I've seen before include "alpine pinnacle" , "see in Paris!" , "French police officer" , "policeman in Paris?" , "French officer - merge DNA (anag)" .)