Medic purchasing green colour scheme (5)
I believe the answer is:
decor
'colour scheme' is the definition.
(I've seen this before)
'medic purchasing green' is the wordplay.
'medic' becomes 'dr'.
'purchasing' means one lot of letters goes inside another (some letters take possession of others).
'green' becomes 'eco' (abbreviation for ecological).
'dr' enclosing 'eco' is 'DECOR'.
(Other definitions for decor that I've seen before include "Internal ornamentation" , "ornament" , "Interior design; stage scenery" , "furnishings required" , "Interior layout and furnishings" .)