The least successful journalist's yarn (7)
I believe the answer is:
worsted
'yarn' is the definition.
(I know that worsted is a type of yarn)
'the least successful journalist's' is the wordplay.
'the least successful' becomes 'worst' (I've seen this in another clue).
'journalist' becomes 'ed' (short for editor).
'worst'+'ed'='WORSTED'
(Other definitions for worsted that I've seen before include "Set word (anag.)" , "Wiped the floor with" , "Beat" , "stuff" , "Beaten" .)