Captain supplied with kit, including helmet (8)
I believe the answer is:
headgear
'helmet' is the definition.
(I know that headgear is a type of helmet)
'captain supplied with kit' is the wordplay.
'captain' becomes 'head' (both can mean a leader).
'supplied with' is a charade indicator (letters next to each other).
'kit' becomes 'gear' (I've seen this before).
'head'+'gear'='HEADGEAR'
'including' acts as a link.
(Other definitions for headgear that I've seen before include "bowler, for example" , "Trilby e.g" , "cap, say" , "crown, say" , "Hats, helmets etc." .)