A drop at a time (4)
I believe the answer is:
fall
I believe this clue is a double definition.
'a drop' is the first definition.
(I know that drop is a more specific form of decline)
'a time' is the second definition.
(I know that fall is a type of hour)
'at' is the link.
(Other definitions for fall that I've seen before include "Rapid descent" , "Autumn - trip" , "Cascade of water" , "Tumble, drop" , "Stumble" .)