On which to cook mountains? (5)
I believe the answer is:
range
'mountains?' is the definition.
(range of mountains)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for range that I've seen before include "Grassland - variety" , "where cattle may graze" , "Scope, extent" , "Selection" , "Full choice of clothes" .)