Simple copper perhaps uncovered nark before Yard (10)
I believe the answer is:
elementary
'simple' is the definition.
('elementary' can be a synonym of 'simple')
'copper perhaps uncovered nark before yard' is the wordplay.
'copper perhaps' becomes 'element' (copper is a kind of element**).
'uncovered' means to look at the middle letters.
'before' is a charade indicator (letters next to each other).
'yard' becomes 'y'.
The middle letters of 'nark' are 'ar'.
'element'+'ar'+'y'='ELEMENTARY'
(Other definitions for elementary that I've seen before include "Simple, introductory" , "No problem" , "Fundamental" , "most things, to Holmes" , "One precursor to 7 [SECONDARY MODERN]" .)