Prince only eating one sort of fish (7)
I believe the answer is:
halibut
'sort of fish' is the definition.
(I know that halibut is a type of fish as food)
'prince only eating one' is the wordplay.
'prince' becomes 'hal' (name for Prince Henry in Shakespeare's histories).
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'eating' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'hal'+'but'='halbut'
'halbut' placed around 'i' is 'HALIBUT'.
(Other definitions for halibut that I've seen before include "Marine food fish" , "Fish found in Bali hut" , "Large flatfish used as food" , "Large flatfish, tasty to eat" , "Swimmer" .)