Decrease in autumn (4)
I believe the answer is:
fall
I believe this clue is a double definition.
'decrease' is the first definition.
(fall is a kind of decrease)
'autumn' is the second definition.
('fall' can be a synonym of 'autumn')
'in' is the link.
(Other definitions for fall that I've seen before include "A season in Vermont?" , "Autumn in the US" , "The American autumn" , "subside" , "rain?" .)