Plant flag on top of Everest (4)
I believe the answer is:
sage
'plant' is the definition.
(sage is a kind of plant)
'flag on top of everest' is the wordplay.
'flag' becomes 'sag' ('sag' can be a synonym of 'flag').
'on top of' says to take the initial letters (I've seen 'on top' mean this).
The first letter of 'everest' is 'e'.
'sag'+'e'='SAGE'
(Other definitions for sage that I've seen before include "Aromatic herb" , "Velvety-leaved herb" , "Wise woman" , "Herb; wise person" , "Herb used in cooking" .)