Partially treating rot to reveal a cave (6)
I believe the answer is:
grotto
'a cave' is the definition.
(I know that cave can be written as grotto)
'partially treating rot to reveal' is the wordplay.
'partially' says the answer is hidden in the clue.
'GROTTO' is hidden in the letters of 'treating rot to reveal'.
(Other definitions for grotto that I've seen before include "Pretty cave" , "Artificially or naturally ornamented cave" , "Small cave as at Lourdes" , "Small cave, perhaps with religious connotations" , "Attractive natural, or artificial, cave" .)