Simple scheme takes in upright character (5)
I believe the answer is:
plain
'simple' is the definition.
(synonyms)
'scheme takes in upright character' is the wordplay.
'scheme' becomes 'plan' (synonyms).
'takes in' indicates putting letters inside.
'upright character' becomes 'I' (vertical letter of the alphabet).
'plan' placed around 'i' is 'PLAIN'.
(Other definitions for plain that I've seen before include "Unprepossessing" , "Basic" , "open country" , "Evident; not fancy" , "Ordinary; area of land" .)