Gosh! Half of that is fiction! (4)
I believe the answer is:
myth
'fiction' is the definition.
(myth is a kind of fiction)
'gosh half of that' is the wordplay.
'gosh' becomes 'my' (both are old-fashioned expressions of surprise).
'half of' means to take half the letters.
'that' cut in half is 'th'.
'my'+'th'='MYTH'
'is' acts as a link.
(Other definitions for myth that I've seen before include "Legend; fable" , "A US president; a UK PM" , "Wildly-held but false belief or idea" , "don't believe a word of it!" , "Widely held misconception" .)