Hard to believe how big the vixen's brush was, they said (4,4)
I believe the answer is:
tall tale
'big the vixen's brush was they said' is the definition.
The definition suggests an adverb but the answer is not.
'hard to believe' is the wordplay.
'hard' becomes 'tall' (similar in meaning).
'to believe' becomes 'tale' (I am not sure about this - if you are sure you should believe this answer much more).
'tall'+'tale'='TALL TALE'
'how' acts as a link.
Can you help me to learn more?
(Other definitions for tall tale that I've seen before include "unbelievable" , "Fanciful story" , "Incredible story" , "Improbable story" , "Story that is difficult to believe, lofty narrative" .)