Hydrogen-filled dark, inflatable vessel (6)
I believe the answer is:
dinghy
'inflatable vessel' is the definition.
(I know this)
'hydrogen-filled dark' is the wordplay.
'hydrogen' becomes 'H' (H is the chemical symbol for hydrogen).
'filled' is an insertion indicator.
'dark' becomes 'dingy' (dingy means dark or gloomy).
'h' placed within 'dingy' is 'DINGHY'.
(Other definitions for dinghy that I've seen before include "Small open boat, may be inflatable" , "Light rowing boat" , "Small rowing or sailing boat" , "Small boat propelled with oars" , "Small boat, may be a rubber inflatable" .)