Cliff's not very small trunks (3)
I believe the answer is:
tor
This clue could be a double definition.
'cliff's' is the first definition.
The definition and answer can be both natural objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'not very small trunks' is the second definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
(Other definitions for tor that I've seen before include "Rocky crag" , "(Dartmoor) rocky hill" , "Rot on a hill in Cornwall" , "Dartmoor peak" , "fell" .)