Muddy border turned over before end of day (4)
I believe the answer is:
miry
'muddy' is the definition.
(synonyms)
'border turned over before end of day' is the wordplay.
'border' becomes 'rim' (both can mean an edge).
'turned over' is a reversal indicator.
'before' is a charade indicator (letters next to each other).
'end of' indicates one should take the final letters.
The final letter of 'day' is 'y'.
'rim' written backwards gives 'mir'.
'mir'+'y'='MIRY'
(Other definitions for miry that I've seen before include "Very boggy" , "Muddy" .)