Captain appearing with sailor's fish (8)
I believe the answer is:
skipjack
'fish' is the definition.
(skipjack is a kind of fish)
'captain appearing with sailor's' is the wordplay.
'captain' becomes 'skip' (I've seen this before).
'appearing with' means one lot of letters go next to another.
'sailor' becomes 'jack' (I've seen this before).
'skip'+'jack'='SKIPJACK'
(Other definitions for skipjack that I've seen before include "Food fish" , "sailing boat" , "Type of tuna" .)