Doctor on a ship - on the rocks perhaps (4)
I believe the answer is:
moss
'the rocks perhaps' is the definition.
I can't judge whether this defines the answer.
'doctor on a ship' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'on' means one lot of letters go next to another.
'a ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'mo'+'ss'='MOSS'
'on' is the link.
Can you help me to learn more?
(Other definitions for moss that I've seen before include "Type of plant growing in damp places" , "material not in Rolling Stones' collection?" , "Veteran racing driver" , "Stirling or Kate?" , "Small plants growing as thick mat" .)