Hill made of piled-up rubbish (3)
I believe the answer is:
tor
'hill' is the definition.
(tor is a kind of hill)
'up rubbish' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'rubbish' becomes 'rot' (I've seen this before).
'rot' back-to-front is 'TOR'.
'made of piled' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tor that I've seen before include "rise" , "It rises" , "A rocky hill in Devon" , "fell" , "Hill or rocky peak" .)