Basic, neat, merely used (10)
I believe the answer is:
elementary
'basic' is the definition.
(similar in meaning)
'neat merely used' is the wordplay.
'used' indicates an anagram.
'neat'+'merely'='neatmerely'
'neatmerely' is an anagram of 'ELEMENTARY'.
(Other definitions for elementary that I've seen before include "most things, to Holmes" , "Rudimentary; introductory" , "description of subatomic particle" , "Principal" , "Easy" .)