Happen to exist before autumn (6)
I believe the answer is:
befall
'happen to' is the definition.
(synonyms)
'exist before autumn' is the wordplay.
'exist' becomes 'be' (synonyms).
'before' says to put letters next to each other.
'autumn' becomes 'fall' (synonyms).
'be'+'fall'='BEFALL'
(Other definitions for befall that I've seen before include "Happen to (someone)" , "Happen, occur" , "Turn out" .)