Sooner or later does he say? Often that's nonsense (3,2,5,4)
I believe the answer is:
one of these days
'sooner or later' is the definition.
(synonyms)
'does he say? often that's nonsense' is the wordplay.
'that's nonsense' indicates anagramming the letters.
'does'+'he'+'say'+'often'='doeshesayoften'
'doeshesayoften' anagrammed gives '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?" , "At some time in the near future" , "Sooner or later" .)