Sooner or later, defy senate and shoo out (3,2,5,4)
I believe the answer is:
one of these days
'sooner or later' is the definition.
('one of these days' can be a synonym of 'sooner or later')
'defy senate and shoo out' is the wordplay.
'and' is a charade indicator (letters next to each other).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'defy'+'senate'+'shoo'='defysenateshoo'
'defysenateshoo' is an anagram of 'ONE OF THESE DAYS'.
(Other definitions for one of these days that I've seen before include "Some time soon" , "provide a selection for a date in the diary?" , "Sooner or later" , "At some time in the near future" .)