Quieten drug smuggler, going around very loudly (6)
I believe the answer is:
muffle
'quieten' is the definition.
(muffling is a kind of quietening)
'drug smuggler going around very loudly' is the wordplay.
'drug smuggler' becomes 'mule' (mule can informally mean a drug smuggler).
'going around' is an insertion indicator.
'very loudly' becomes 'ff' (short for fortissimo, 'very loud' in music).
'mule' placed around 'ff' is 'MUFFLE'.
(Other definitions for muffle that I've seen before include "damp" , "Envelop" , "Smother sound" , "Suppress sound as with silencer" , "Keep warm" .)