A letter hard to believe to any extent whatever (2,3)
I believe the answer is:
at all
'whatever' is the definition.
(I've seen this in another clue)
'a letter hard to believe to any extent' is the wordplay.
I cannot quite understand how this works, but
'a' is present in the answer.
'letter' could be 't' and 't' is located in the answer.
'any' could be 'all' (similar in meaning) and 'all' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for at all that I've seen before include "Whatsoever" , "in any circumstances" , "in any way" .)