Board English vessel carrying doctor (6)
I believe the answer is:
embark
'board' is the definition.
(embarking is a kind of boarding)
'english vessel carrying doctor' is the wordplay.
'english' becomes 'e' (abbreviation).
'vessel' becomes 'ark' (ark is a kind of vessel).
'carrying' means one lot of letters go next to another (in a down clue, the bottom letters carry others).
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'ark' put after 'mb' is 'mbark'.
'e'+'mbark'='EMBARK'
(Other definitions for embark that I've seen before include "Join ship" , "Set out on journey" , "get on board" , "Put to sea" , "Go aboard" .)