Plant doctor on ship (4)
I believe the answer is:
moss
'plant' is the definition.
(I've seen this in other clues)
'doctor on ship' is the wordplay.
'doctor' becomes 'mo' (medical officer**).
'on' means one lot of letters go next to another.
'ship' becomes 'ss'.
'mo'+'ss'='MOSS'
(Other definitions for moss that I've seen before include "Plant(s) not wanted on lawns" , "Spore-bearing plant; bog" , "Small flowerless green plant without proper roots" , "Small plants growing as thick mat" , "Spore-producing plant" .)