Rubbish in grotto (3)
I believe the answer is:
rot
'rubbish' is the definition.
(both can mean nonsense)
'in grotto' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ROT' is hidden within 'grotto'.
(Other definitions for rot that I've seen before include "Rubbish (colloq.)" , "Corruption" , "Rhubarb" , "Balderdash" , "bunk" .)