Primary ABC is revised (5)
I believe the answer is:
basic
'primary' is the definition.
(I've seen this before)
'abc is revised' is the wordplay.
'revised' is an anagram indicator.
'abc'+'is'='abcis'
'abcis' is an anagram of 'BASIC'.
(Other definitions for basic that I've seen before include "Plain" , "Elementary - without extras" , "Fundamental instruction's" , "Without frills" , "Fundamental; alkaline" .)