Ignition device in vessel (7)
I believe the answer is:
lighter
I believe this is a double definition.
'ignition device' is the first definition.
(device for igniting or lighting something)
'vessel' is the second definition.
(lighter is a kind of vessel)
'in' acts as a link.
(Other definitions for lighter that I've seen before include "Flat-bottomed boat" , "Boat for heavy loads and fire igniter" , "Ship to shore boat" , "more blonde" , "Cargo barge; smoker's accessory" .)