Go off hill climbing (3)
I believe the answer is:
rot
'go off' is the definition.
(I've seen this before)
'hill climbing' is the wordplay.
'hill' becomes 'tor' (tor is a kind of hill).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'tor' reversed gives 'ROT'.
(Other definitions for rot that I've seen before include "Break down, waste away" , "Rubbish (colloq.)" , "Rhubarb" , "Putrefy" , "Decay - rubbish" .)