Cook fish slices in great quantities (6)
I believe the answer is:
freely
'in great quantities' is the definition.
(as in drinking freely)
'cook fish slices' is the wordplay.
'cook' becomes 'fry' (frying is a type of cooking).
'fish' becomes 'eel' (eel is a type of fish).
'slices' indicates putting letters inside (some letters slice into others).
'fry' going around 'eel' is 'FREELY'.
(Other definitions for freely that I've seen before include "Without compulsion" , "boldly" , "in copious quantities" , "easily" , "Without restriction or payment" .)