Talk drugs, mustard and marsh, say? (5)
I believe the answer is:
gases
'mustard and marsh say?' is the definition.
(mustard gas and marsh gas are types of gas)
'talk drugs' is the wordplay.
'talk' becomes 'gas' (I've seen this before**).
'drugs' becomes 'es' (E is a term for an ecstasy pill).
'gas'+'es'='GASES'
(Other definitions for gases that I've seen before include "rabbits" , "Such as radon and neon" , "Cruelly kills" , "Eg, O, H, Ne, CO, etc." , "Neon, oxygen, etc" .)