Annual drop (4)
I believe the answer is:
fall
This clue might be a double definition.
'annual' is the first definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'drop' is the second definition.
(I know that drop is a kind of decline)
Can you help me to learn more?
(Other definitions for fall that I've seen before include "Rapid descent" , "Autumn in the USA" , "A season in the US" , "Drop; autumn" , "Sudden decline" .)