Drop in autumn (4)
I believe the answer is:
fall
I believe this clue is a double definition.
'drop' is the first definition.
(synonyms)
'autumn' is the second definition.
(synonyms)
'in' is the link.
(Other definitions for fall that I've seen before include "Autumn - trip" , "Collapse; decline" , "Autumn in the US" , "Stateside season" , "Autumn - collapse" .)