Manifest one in drawing (5)
I believe the answer is:
plain
'manifest' is the definition.
(synonyms)
'one in drawing' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'drawing' becomes 'plan' (plan is a kind of drawing).
'i' inserted inside 'plan' is 'PLAIN'.
(Other definitions for plain that I've seen before include "Unattractive - flat" , "lament" , "Simple, unadorned" , "Straightforward" , "no oil painting features" .)