Surrender captain on a vessel (4,2)
I believe the answer is:
cave in
'a vessel' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'surrender captain' is the wordplay.
'surrender' becomes 'cave' (I've seen this in another clue).
'captain' becomes 'in' (I can't explain this - if you can you should believe this answer much more).
'cave'+'in'='CAVE-IN'
'on' acts as a link.
Can you help me to learn more?
(Other definitions for cave in that I've seen before include "(Opposition, roof) collapse" , "(Roof) collapse; abandon resistance" , "Give way" , "Subside, collapse" , "Yield to outside pressure" .)