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