In which to boil water or cook fish (6)
I believe the answer is:
kettle
'in which to boil water or cook fish' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for kettle that I've seen before include "One boiled" , "Check elements causing trouble in demonstration" , "Kitchen boiler?" , "Domestic appliance that can whistle?" , "Spouted vessel" .)