Seafood king caught in Hackney? (4)
I believe the answer is:
crab
'seafood' is the definition.
(crab is a kind of seafood)
'king caught in hackney?' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'caught in' indicates putting letters inside.
'hackney?' becomes 'cab' (I've seen this in other clues).
'r' placed into 'cab' is 'CRAB'.
(Other definitions for crab that I've seen before include "Ten-legged shellfish" , "winch" , "Sourpuss < to wreck" , "Clawed shellfish" , "one is a fiddler" .)