Disintegrate - autumn has a role (4,5)
I believe the answer is:
fall apart
'disintegrate' is the definition.
(I know this)
'autumn has a role' is the wordplay.
'autumn' becomes 'fall' (synonyms).
'has' is a charade indicator (letters next to each other).
'role' becomes 'part' (eg a part or role in a film).
'fall'+'a'+'part'='FALL APART'
(Other definitions for fall apart that I've seen before include "Go to pieces" , "Disintegrate" , "Come to pieces" .)