Really? That’s half gone? It’s not true (4)
I believe the answer is:
myth
'it's not true' is the definition.
(I've seen this in another clue)
'really? that's half gone?' is the wordplay.
'really?' becomes 'my' (I am not sure about this - if you are sure you should believe this answer much more).
'half gone?' suggests halving the letters.
'that' halved is 'th'.
'my'+'th'='MYTH'
Can you help me to learn more?
(Other definitions for myth that I've seen before include "a false notion" , "Fanciful story" , "Story of old gods" , "Legend; fable" , "Invention" .)