Areas containing unknown meadow plant (6)
I believe the answer is:
azalea
'plant' is the definition.
(type of plant)
'areas containing unknown meadow' is the wordplay.
'areas' becomes 'AA' (two abbreviations for area).
'containing' indicates putting letters inside.
'unknown' becomes 'z' (algebra).
'meadow' becomes 'lea' (a lea is a grassy area).
'aa' placed around 'z' is 'aza'.
'aza'+'lea'='AZALEA'
(Other definitions for azalea that I've seen before include "Rhododendron relative" , "Rhododendron-like shrub" , "Bright-flowered shrub" , "Ericaceous plant" , "Shrub with brightly coloured flowers" .)