Hill rubbish must be tipped (3)
I believe the answer is:
tor
'hill' is the definition.
(I know that tor is a type of hill)
'rubbish must be tipped' is the wordplay.
'rubbish' becomes 'rot' (both can mean nonsense).
'must be tipped' says the letters should be written backwards.
'rot' reversed gives 'TOR'.
(Other definitions for tor that I've seen before include "Rot on rocky peak" , "Feature of Dartmoor" , "(Dartmoor) rocky hill" , "Rocky height, eg Mam" , "A rocky hill in Devon" .)