Cook fish within, without compulsion (6)
I believe the answer is:
freely
'without compulsion' is the definition.
(I've seen this before)
'cook fish within' is the wordplay.
'cook' becomes 'fry' (frying is a type of cooking).
'fish' becomes 'eel' (eel is a type of fish).
'within' is an insertion indicator.
'fry' enclosing 'eel' is 'FREELY'.
(Other definitions for freely that I've seen before include "Liberally; willingly" , "Without restriction or payment" , "easily" , "boldly" , "Without let or hindrance" .)