Cavities in use on board vessel (7)
I believe the answer is:
sinuses
'cavities' is the definition.
(sinus is a kind of cavity)
'in use on board vessel' is the wordplay.
'on board' is an insertion indicator (inserted letters are taken on board).
'vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
'in'+'use'='inuse'
'inuse' put into 'ss' is 'SINUSES'.
(Other definitions for sinuses that I've seen before include "holes in the head" , "Top spaces" , "Empty spaces" , "Cavities in skull connecting with nostrils" .)