Captain discards small fish (6)
I believe the answer is:
kipper
'fish' is the definition.
(type of fish)
'captain discards small' is the wordplay.
'captain' becomes 'skipper' (synonyms).
'discards' suggests deleting specific letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'skipper' with 's' removed is 'KIPPER'.
(Other definitions for kipper that I've seen before include "Preserved herring" , "One yet to wake" , "Cured fish (one having forty winks?)" , "has been caught napping" , "One smoked" .)