Doctor, on ship, finds plant life (4)
I believe the answer is:
moss
'plant life' is the definition.
'moss' can be an answer for 'plant' (I've seen this before). I'm not certain of the 'life' bit.
'doctor on ship' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'mo'+'ss'='MOSS'
'finds' acts as a link.
Can you help me to learn more?
(Other definitions for moss that I've seen before include "Shape of pointed arch" , "Veteran racing driver" , "Plant growing on many lawns" , "Parasitic plant" , "Damp-loving plant" .)