Break down in grotto (3)
I believe the answer is:
rot
'break down' is the definition.
(I've seen this in another clue)
'in grotto' is the wordplay.
'in' indicates a hidden word.
'ROT' can be found hidden inside 'grotto'.
(Other definitions for rot that I've seen before include "Decay or disintegrate" , "Decay, wither" , "Putrefy" , "'Decay, decompose (3)'" , "Decay in grotesque way" .)