With "Nonsense!" mount up (3)
I believe the answer is:
rot
'with nonsense' is the definition.
'rot' can be an answer for 'nonsense' (rot can mean nonsense or rubbish). I'm unsure of the 'with' bit.
'mount up' is the wordplay.
'mount' becomes 'tor' (I've seen this in other clues).
'up' is a reversal indicator (in down clue: letters go upwards).
'tor' backwards is 'ROT'.
Can you help me to learn more?
(Other definitions for rot that I've seen before include "Rubbish (colloq.)" , "Decay, moulder" , "Decay, go bad" , "Break down, waste away" , "Putrefy" .)