Go off climbing — this peak? (3)
I believe the answer is:
tor
'this peak?' is the definition.
(a tor is a rocky peak)
'go off climbing' is the wordplay.
'go off' becomes 'rot' (as in food going off).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'rot' in reverse letter order is 'TOR'.
(Other definitions for tor that I've seen before include "Rocky prominence" , "rise" , "Rocky crag" , "Dartmoor peak" , "Feature of Dartmoor" .)