Rocky outcrop stuck up? Nonsense! (3)
I believe the answer is:
rot
'nonsense' is the definition.
(rot can mean nonsense or rubbish)
'rocky outcrop stuck up?' is the wordplay.
'rocky outcrop' becomes 'tor' (term for a hill).
'stuck up?' says the letters should be written backwards.
'tor' back-to-front is 'ROT'.
(Other definitions for rot that I've seen before include "Decay, wither" , "Gradual decline" , "Putrefy" , "Rubbish (colloq.)" , "Collapse" .)