Some tales La Fontaine rejected could be untrue (5)
I believe the answer is:
false
'be untrue' is the definition.
('false' can be similar in meaning to 'untrue')
'some tales la fontaine rejected' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'rejected' says the letters should be written backwards.
'eslaf' is hidden in the letters of 'tales la fontaine'.
'eslaf' in reverse letter order is 'FALSE'.
'could' is the link.
(Other definitions for false that I've seen before include "Misleading" , "Forged" , "Bogus" , "Artificial" , "Spurious, untrue" .)