Descending a hill (4)
I believe the answer is:
down
I believe this clue is a double definition.
'descending' is the first definition.
(associated in meaning)
'a hill' is the second definition.
(I've seen this in other clues)
(Other definitions for down that I've seen before include "Some 13 Acrosses" , "Fuzz" , "Immediately" , "Soft fine feathers" , "See 1" .)