Heroin stashed in dirty vessel (6)
I believe the answer is:
dinghy
'vessel' is the definition.
(dinghy is a kind of vessel)
'heroin stashed in dirty' is the wordplay.
'heroin' becomes 'H' (synonyms).
'stashed in' indicates putting letters inside.
'dirty' becomes 'dingy' (similar in meaning).
'h' inserted inside 'dingy' is 'DINGHY'.
(Other definitions for dinghy that I've seen before include "Small open boat, maybe a rubber inflatable" , "Small rowing or sailing boat" , "Small boat propelled with oars" , "Light rowing boat" , "Small open boat, may be inflatable" .)