Fish in shop, a halibut (4)
I believe the answer is:
opah
'fish' is the definition.
(opah is a kind of fish)
'in shop a halibut' is the wordplay.
'in' indicates the answer is hidden within the clue.
'OPAH' can be found hidden inside 'shop a halibut'.
(Other definitions for opah that I've seen before include "Kingfish" , "Ocean swimmer" , "Large deep-sea fish" .)