Cave __ (3)
'cave' is the definition.
(I know that cave can be written as grotto)
This is all the clue.
(Other definitions for grotto that I've seen before include "Small cave as at Lourdes" , "It's left on Dec 24 by Santa and his reindeer" , "Pretty cave" , "(Imitation) cave" , "A picturesque cave as in Lourdes (6)" .)
(Other definitions for cavern that I've seen before include "Vast dark space" , "Large cave" , "Detection device" , "Grotto" , "Large dark hole in the ground" .)
(Other definitions for art that I've seen before include "The works of Alma-Tadema" , "Trickery" , "Eg, paintings, sculpture" , "eg Painting" , "Works in a gallery" .)