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 a hidden word.
'ROT' is hidden within 'grotto'.
(Other definitions for rot that I've seen before include "Rubbish (colloq.)" , "Corruption" , "Rhubarb" , "Balderdash" , "bunk" .)