Professional on a sailing vessel (4)
I believe the answer is:
proa
'sailing vessel' is the definition.
(type of boat)
'professional on a' is the wordplay.
'professional' becomes 'pro' (synonyms).
'on' means one lot of letters go next to another.
'pro'+'a'='PROA'
(Other definitions for proa that I've seen before include "Canoe-like boat of South Pacific" , "Vessel" , "Boat's" , "Malay boat with identical ends and one outrigger" , "Malay sailing boat with outrigger" .)