Plain art could be evergreen (3,5)
I believe the answer is:
air plant
'evergreen' is the definition.
The definition and answer can be both plants as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'plain art could be' is the wordplay.
'could be' indicates anagramming the letters.
'plain'+'art'='plainart'
'plainart' anagrammed gives 'AIR-PLANT'.
Can you help me to learn more?
(Other definitions for air plant that I've seen before include "isn't rooted to the ground" , "Epiphyte taking nourishment from the atmosphere" , "Epiphyte absorbing nutrients from the atmosphere" .)