Extremely bad article with entire ending missing (5)
I believe the answer is:
awful
'extremely bad' is the definition.
(awful means very bad)
'article with entire ending missing' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'with' becomes 'w' (abbreviation).
'entire' becomes 'full' (synonyms).
'ending missing' means to remove the last letter.
'full' with its final letter taken away is 'ful'.
'a'+'w'+'ful'='AWFUL'
(Other definitions for awful that I've seen before include "Dreadful - spine-chilling" , "Not good at all" , "Horrific" , "Really terrible, ghastly" , "pants" .)