As a matter of fact, this can't be spoken (7)
I believe the answer is:
untruth
'as a matter of fact this can't be spoken' is the definition.
The definition suggests a singular noun which matches the answer.
'fact this can't be spoken' is the wordplay.
I cannot really understand how this works, but
'fact' could be 'truth' (truth is a kind of fact) and 'truth' is present in the answer.
The remaining letters 'un' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for untruth that I've seen before include "story" , "A lie or false statement" , "A genuinely deniable" , "Fib" , "Falsehood or lie" .)