Right with union at eastern plant (3)
I believe the answer is:
rue
'plant' is the definition.
(type of shrub)
'right with union at eastern' is the wordplay.
'right' becomes 'r' (common abbreviation).
'with' says to put letters next to each other.
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'at' is a charade indicator (letters next to each other).
'eastern' becomes 'e'.
'r'+'u'+'e'='RUE'
(Other definitions for rue that I've seen before include "Feel regret for the herb" , "Deplore" , "Nice way" , "French way" , "Regret or repent" .)