Area twice invaded by unknown meadow plant (6)
I believe the answer is:
azalea
'plant' is the definition.
(type of plant)
'area twice invaded by unknown meadow' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'twice' says to double the letters.
'invaded by' means one lot of letters goes inside another.
'unknown' becomes 'z' (algebra).
'meadow' becomes 'lea' (a lea is a grassy area).
'a' duplicated is 'aa'.
'aa' going around 'z' is 'aza'.
'aza'+'lea'='AZALEA'
(Other definitions for azalea that I've seen before include "Acid-loving plant" , "Rhododendron relative" , "Rhododendron-like shrub" , "Deciduous shrub" , "Showy flowering shrub" .)