Mess left after meal the day before (8)
I believe the answer is:
dishevel
'mess' is the definition.
(I've seen this before)
'left after meal the day before' is the wordplay.
'left' becomes 'l' (common abbreviation).
'after' is a charade indicator (letters next to each other).
'meal' becomes 'dish' (a dish is a particular meal).
'the day before' becomes 'eve' (eve is the night before something).
'dish'+'eve'='disheve'
'l' put after 'disheve' is 'DISHEVEL'.
(Other definitions for dishevel that I've seen before include "Make untidy" , "Ruffle (e.g. hair)" , "get into a mess" , "Disarrange - rumple" , "Get ruffled" .)