Check when boarding earlier vessel (8)
I believe the answer is:
schooner
'vessel' is the definition.
(type of ship)
'check when boarding earlier' is the wordplay.
'check' becomes 'ch' (abbreviation).
'when boarding' indicates putting letters inside.
'earlier' becomes 'sooner' (synonyms).
'ch' put within 'sooner' is 'SCHOONER'.
(Other definitions for schooner that I've seen before include "Old sailing vessel and large glass" , "A measure or glass for sherry" , "Item used for drinking" , "Swift sailing vessel - sherry glass" , "that carries sherry" .)