Gosh! That’s not half a made-up story (4)
I believe the answer is:
myth
'a made-up story' is the definition.
'myth' can be an answer for 'story' (myth is a kind of story). I'm unsure of the 'a made-up' bit.
'gosh that's not half' is the wordplay.
'gosh' becomes 'my' (both are old-fashioned expressions of surprise).
'not half' suggests halving the letters.
'that' with half the letters taken is 'th'.
'my'+'th'='MYTH'
Can you help me to learn more?
(Other definitions for myth that I've seen before include "false interpretation?" , "Ancient story" , "Traditional allegory" , "Illusion" , "Widely held misconception" .)