Imagine it's not plain (5)
I believe the answer is:
fancy
I believe this is a double definition.
'imagine' is the first definition.
(I know that fancy is a more specific form of the action imagine)
'not plain' is the second definition.
('fancy' is the opposite to 'plain')
'it's' acts as a link.
(Other definitions for fancy that I've seen before include "Imagine - whim - highly decorated" , "Imagine; expensive" , "Elaborate; whim" , "Imagine - find attractive" , "Unfounded belief" .)