Sad after autumn trip? (4,4)
I believe the answer is:
fall down
'trip?' is the definition.
(thesaurus)
'sad after autumn' is the wordplay.
'sad' becomes 'down' (I've seen this before).
'after' says to put letters next to each other.
'autumn' becomes 'fall' (synonyms).
'down' put after 'fall' is 'FALL DOWN'.
(Other definitions for fall down that I've seen before include "Collapse" , "Tumble" .)