The key question is what to cook on (7)
I believe the answer is:
griddle
'cook on' is the definition.
'griddle' can be an answer for 'cook' (to cook with a griddle). I am not sure about the 'on' bit.
'the key question is what' is the wordplay.
I cannot really understand how this works, but
'key' could be 'g' (musical key) and 'g' is found in the answer.
'question' could be 'riddle' (riddle is a kind of question) and 'riddle' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for griddle that I've seen before include "Iron plate for cooking on" , "cooking utensil" , "Is it a hot surface" , "cooking accessory" , "Cook" .)