Hammerhead in gloomy boat (6)
I believe the answer is:
dinghy
'boat' is the definition.
(type of boat)
'hammerhead in gloomy' is the wordplay.
'hammerhead' becomes 'h' (head letter of 'hammer').
'in' indicates putting letters inside.
'gloomy' becomes 'dingy' (both can mean dark or drab).
'h' put into 'dingy' is 'DINGHY'.
(Other definitions for dinghy that I've seen before include "Small rowing or sailing boat" , "Small boat propelled with oars" , "vessel" , "Small boat, may be a rubber inflatable" , "Small open boat, may be inflatable" .)