Operator accepts source of tuna fish (8)
I believe the answer is:
sturgeon
'fish' is the definition.
(sturgeon is a kind of fish)
'operator accepts source of tuna' is the wordplay.
'operator' becomes 'surgeon' (I've seen this before).
'accepts' indicates putting letters inside.
'source of' suggests taking the first letters.
The initial letter of 'tuna' is 't'.
'surgeon' placed around 't' is 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "supplier of delicacy" , "Egg producer" , "producer of expensive eggs" , "Fish valued for caviar" , "Our gents, oddly, caught the fish" .)