Posh leaving desirable London area (6)
I believe the answer is:
poplar
'london area' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for poplar that I've seen before include "Timber tree" , "Deciduous tree" , "The tree that quivers" , "Slender tree with quivering leaves" , "Tall, slender tree" .)