Autumn trip (4)
I believe the answer is:
fall
This clue is a double definition.
'autumn' is the first definition.
(synonyms)
'trip' is the second definition.
(trip can mean to fall over)
(Other definitions for fall that I've seen before include "sink" , "Drop, decrease" , "A season in the US" , "Stumble" , "Autumn in the US" .)