Rubbish in the grotto (3)
I believe the answer is:
rot
'rubbish' is the definition.
(I've seen this before)
'in the grotto' is the wordplay.
'in' indicates a hidden word.
'ROT' is hidden within 'grotto'.
(Other definitions for rot that I've seen before include "crumble" , "'Decay, decompose (3)'" , "Balderdash" , "Paradise" , "Collapse" .)