King boarding large sailing vessel (4)
I believe the answer is:
brig
'sailing vessel' is the definition.
(I know that brig is a type of sailing vessel)
'king boarding large' is the wordplay.
'king' becomes 'R' (abbreviation of Latin rex).
'boarding' is an insertion indicator.
'large' becomes 'big' (synonyms).
'r' put inside 'big' is 'BRIG'.
(Other definitions for brig that I've seen before include "'Two-masted,square-rigged ship (4)'" , "Sailing-vessel" , "Two-masted square rigger" , "A ship; army officer (abbr.)" , "senior officer briefly used" .)