Possibly Dartmoor's big mound of rubbish piled up (3)
I believe the answer is:
tor
'possibly dartmoor's big mound' is the definition.
(term for a rocky mound)
'rubbish piled up' is the wordplay.
'rubbish' becomes 'rot' (both can mean nonsense).
'piled up' shows that the letters should be reversed in order.
'rot' written backwards gives 'TOR'.
'of' is the link.
(Other definitions for tor that I've seen before include "rise" , "Rot on a Cornish hill" , "Rot on a hill in Cornwall" , "Rot on rocky peak" , "feature of Glastonbury" .)