It's impossible, climbing or descending (3,2)
I believe the answer is:
not on
'it's impossible' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for not on that I've seen before include "Colloquially, cannot be tolerated or accepted" , "Informally, cannot be permitted or tolerated" , "It's unfeasible" , "Modern phrase for unacceptable" , "Common expression meaning won't be entertained or tolerated" .)