Vessel more willingly boarded by companion (8)
I believe the answer is:
schooner
'vessel' is the definition.
(schooner is a kind of vessel)
'more willingly boarded by companion' is the wordplay.
'more willingly' becomes 'sooner' (as in phrases like 'I'd sooner die').
'boarded by' is an insertion indicator (I've seen this in other clues).
'companion' becomes 'ch' (I've seen this in other clues).
'sooner' placed around 'ch' is 'SCHOONER'.
(Other definitions for schooner that I've seen before include "Swift sailing vessel - sherry glass" , "Two-masted sailing ship" , "Old sailing ship and tall sherry glass" , "that carries sherry" , "Type of glass" .)