Loose woman putting last of opium in gin (5)
I believe the answer is:
tramp
'loose woman' is the definition.
'last of opium in gin' is the wordplay.
'last of' says to take the final letters.
'in' indicates putting letters inside.
'gin' becomes 'trap' (I've seen this before).
The last letter of 'opium' is 'm'.
'm' inserted within 'trap' is 'TRAMP'.
'putting' acts as a link.
(Other definitions for tramp that I've seen before include "woman who's promiscuous?" , "cargo-boat" , "Bum!" , "cargo boat" , "Walk heavily like hobo" .)