Protein-rich food, case of prize fish turned up (6)
I believe the answer is:
peanut
'protein-rich food' is the definition.
I can't judge whether this definition defines the answer.
'case of prize fish turned up' is the wordplay.
'case of' suggests removing the centre (outsides of).
'fish' becomes 'tuna' (tuna is a kind of fish**).
'turned up' is a reversal indicator.
'prize' with its middle removed is 'pe'.
'tuna' backwards is 'anut'.
'pe'+'anut'='PEANUT'
Can you help me to learn more?
(Other definitions for peanut that I've seen before include "Salted or dry roasted item" , "Bar nibble" , "Something to eat" , "(Plant with) edible underground seed" , "It ripens in a vine pod underground" .)