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