Young hare heading off round back of this mountain (7)
I believe the answer is:
everest
'mountain' is the definition.
(Mount Everest)
'young hare heading off round back of this' is the wordplay.
'young hare' becomes 'leveret' (I've seen this before).
'heading off' means to remove the first letter.
'round' means one lot of letters goes inside another.
'back of' says to take the final letters.
The final letter of 'this' is 's'.
'leveret' with its initial letter taken off is 'everet'.
'everet' going around 's' is 'EVEREST'.
(Other definitions for everest that I've seen before include "Tee revs up the mountain" , "Himalyan peak" , "Mountain named after a British surveyor general of India" , "Mount, climbed 1953" , "A mountain to climb" .)