Enemy later beaten, this being simple (10)
I believe the answer is:
elementary
'this being simple' is the definition.
('elementary' can be a synonym of 'simple')
'enemy later beaten' is the wordplay.
'beaten' is an anagram indicator.
'enemy'+'later'='enemylater'
'enemylater' with letters rearranged gives 'ELEMENTARY'.
(Other definitions for elementary that I've seen before include "schooling?" , "One precursor to 7 [SECONDARY MODERN]" , "Basic, simple" , "Primary" , "description of subatomic particle" .)