Plant doctor on vessel (4)
I believe the answer is:
moss
'plant' is the definition.
(I've seen this in other clues)
'doctor on vessel' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'on' is a charade indicator (letters next to each other).
'vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
'mo'+'ss'='MOSS'
(Other definitions for moss that I've seen before include "Plant growing on many lawns" , "Damp-loving plant" , "Low-growing plant" , "Small plants growing as thick mat" , "Parasitic plant" .)