Plants unknown in drained area beside meadows (7)
I believe the answer is:
azaleas
'plants' is the definition.
The answer is an example.
'unknown in drained area beside meadows' is the wordplay.
'unknown' becomes 'z' (algebra).
'in' means one lot of letters goes inside another.
'drained' means to remove the middle letters.
'beside' is a charade indicator (letters next to each other) (I've seen this in other clues).
'meadows' becomes 'leas' (lea can mean a meadow or field).
'area' with its middle taken out is 'aa'.
'z' going inside 'aa' is 'aza'.
'aza'+'leas'='AZALEAS'
(Other definitions for azaleas that I've seen before include "Smaller rhododendrons" , "Flowering shrubs" .)