Fake news’s ending poor actor (4)
I believe the answer is:
sham
'fake' is the definition.
(sham can mean fake or pretend)
'news's ending poor actor' is the wordplay.
'ending' indicates one should take the final letters (the ending of the word).
'poor actor' becomes 'ham' (ham can mean a bad or over-the-top actor).
The last letter of 'newss' is 's'.
's'+'ham'='SHAM'
(Other definitions for sham that I've seen before include "Mash, being false" , "'Fake, pseudo (4)'" , "Pretense; bogus" , "False and phoney" , "Phoney sort of hams" .)