With "Nonsense!" mount up (3)
'with nonsense' is the definition.
'rot' can be an answer for 'nonsense' (I've seen this before). I am not certain of the 'with' bit.
'mount up' is the wordplay.
'mount' becomes 'tor' (I've seen this in other clues).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'tor' reversed gives 'ROT'.
Can you help me to learn more?
(Other definitions for rot that I've seen before include "Informal nonsense", "Deteriorate", "Break down, waste away", "Decay, wither", "Decompose; rubbish".)