Mess is complete after carnival emptied out (7)
I believe the answer is:
clutter
'mess' is the definition.
(I've seen this before)
'complete after carnival emptied out' is the wordplay.
'complete' becomes 'utter' (I've seen this before).
'after' means one lot of letters go next to another.
'emptied out' suggests removing the centre (the inside letters are emptied out).
'carnival' with its centre removed is 'cl'.
'utter' after 'cl' is 'CLUTTER'.
'is' acts as a link.
(Other definitions for clutter that I've seen before include "Disorderly mess" , "Confused muddle of things" , "Things lying about untidily" , "Untidy state" , "Disorderly heap" .)