Trip when autumn is finished (4,4)
I believe the answer is:
fall over
'trip' is the definition.
(I've seen this before)
'autumn is finished' is the wordplay.
'autumn' becomes 'fall' (synonyms).
'is finished' becomes 'over' (over can mean finished or ended).
'fall'+'over'='FALL OVER'
'when' is the link.
(Other definitions for fall over that I've seen before include "Trip" , "collapse" , "Take a tumble" .)