Mislays sole arrangement on Sunday (5)
I believe the answer is:
loses
'mislays' is the definition.
('lose' can be a synonym of 'mislay')
'sole arrangement on sunday' is the wordplay.
'arrangement' indicates anagramming the letters.
'on' is a charade indicator (letters next to each other).
'sunday' becomes 's' ().
'sole' with letters rearranged gives 'lose'.
'lose'+'s'='LOSES'
(Other definitions for loses that I've seen before include "Is unable to find" , "Misoplaces" , "Misplaces" , "Mislays or is defeated" , "Fails to win" .)