Fake news: pleas FA regularly rejected (5)
I believe the answer is:
false
'fake' is the definition.
(synonyms)
'news pleas fa regularly rejected' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'rejected' shows that the letters should be reversed in order.
'news'+'pleas'+'fa'='newspleasfa'
The alternating letters of 'newspleasfa' are 'eslaf'.
'eslaf' backwards is 'FALSE'.
(Other definitions for false that I've seen before include "Spurious, untrue" , "Not in accordance with fact" , "Mendacious" , "Incorrect" , "Not genuine or real" .)