Reason egghead left plant... (9)
I believe the answer is:
groundsel
'plant' is the definition.
The answer is an example.
'reason egghead left' is the wordplay.
'reason' becomes 'grounds' (grounds or justification for a particular action).
'egghead' becomes 'e' (head letter of 'egg').
'left' becomes 'l' (common abbreviation).
'grounds'+'e'+'l'='GROUNDSEL'
(Other definitions for groundsel that I've seen before include "Plant of daisy family" , "feed to the birds" , "Common weed" , "plant, unwanted?" .)