A clear scheme includes one (5)
I believe the answer is:
plain
'a clear' is the definition.
(I've seen this before)
'scheme includes one' is the wordplay.
'scheme' becomes 'plan' (I've seen this before).
'includes' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'plan' placed around 'i' is 'PLAIN'.
(Other definitions for plain that I've seen before include "Straightforward" , "lacking special beauty" , "Sheer; frank" , "Obvious - ordinary" , "Not complicated" .)