One criticising terribly easy yarn (8)
I believe the answer is:
naysayer
'one criticising' is the definition.
(someone who rejects or criticises something)
'terribly easy yarn' is the wordplay.
'terribly' is an anagram indicator.
'easy'+'yarn'='easyyarn'
'easyyarn' anagrammed gives 'NAYSAYER'.
(Other definitions for naysayer that I've seen before include "He refuses to agree" , "One who denies" , "Yes, Aryan may be person who habitually denies or refuses" , "Obstructionist" .)