A sort of ABC is fundamental (5)
I believe the answer is:
basic
'fundamental' is the definition.
(synonyms)
'a sort of abc is' is the wordplay.
'a sort of' indicates anagramming the letters.
'abc'+'is'='abcis'
'abcis' with letters rearranged gives 'BASIC'.
(Other definitions for basic that I've seen before include "At the lowest level" , "Fundamental; alkaline" , "Fundamental, forming a starting point" , "Radical" , "Staple; ordinary" .)