Stepfather is a ship's officer (6,4)
I believe the answer is:
second mate
This clue could be a double definition.
'stepfather' is the first definition.
Both the definition and answer are singular nouns.
Perhaps there's a link between them I don't understand?
'a ship's officer' is the second definition.
(another term for the second officer of a ship)
'is' acts as a link.
Can you help me to learn more?
(Other definitions for second mate that I've seen before include "one involved in bigamy?" , "Craftsman, perhaps" , "Naval officer" .)