That is nothing for a fool to admit (2,3)
I believe the answer is:
to wit
'that is' is the definition.
('to wit' can mean 'namely' or 'that is')
'nothing for a fool to admit' is the wordplay.
I cannot quite see how this works, but
'nothing' could be 'o' (O resembles 0) and 'o' is found within the answer.
'fool' could be 'twit' (both can mean a stupid person) and 'twit' is present in the leftover letters.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
(Other definitions for to wit that I've seen before include "That is to say" , "In other words" , "what to do with a broken-down car!" , "Namely" , "i.e." .)