Good hill rising to cave (6)
I believe the answer is:
grotto
'cave' is the definition.
(I know that grotto is a type of cave)
'good hill rising to' is the wordplay.
'good' becomes 'g' (abbreviation).
'hill' becomes 'tor' (tor is a kind of hill).
'rising' says the letters should be written in reverse (in a down clue, letters go up).
'tor' reversed gives 'rot'.
'g'+'rot'+'to'='GROTTO'
(Other definitions for grotto that I've seen before include "It's left on Dec 24 by Santa and his reindeer" , "fancy underground area" , "Small cave as at Lourdes" , "A picturesque cave as in Lourdes (6)" , "(Imitation) cave" .)