Board rescue vessel after English doctor (6)
I believe the answer is:
embark
'board' is the definition.
(embarking is a kind of boarding)
'rescue vessel after english doctor' is the wordplay.
'rescue vessel' becomes 'ark' (rescue ship in the Bible).
'after' is a charade indicator (letters next to each other).
'english' becomes 'e' (abbreviation).
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'e'+'mb'='emb'
'ark' put after 'emb' is 'EMBARK'.
(Other definitions for embark that I've seen before include "Start" , "Go on board ship or plane" , "Put to sea" , "Go aboard" , "Go on board, boat or plane" .)