Cutting down on nice drug? Preposterous! (8)
I believe the answer is:
reducing
'cutting down on' is the definition.
('reduce' can be a synonym of 'cut down on')
'nice drug? preposterous' is the wordplay.
'preposterous' indicates anagramming the letters.
'nice'+'drug'='nicedrug'
'nicedrug' anagrammed gives 'REDUCING'.
(Other definitions for reducing that I've seen before include "Making smaller" , "Bringing down, lowering" , "Cutting down" .)