A right turn-off, corporation's tree plantation (9)
I believe the answer is:
arboretum
'tree plantation' is the definition.
(an arboretum is a garden with trees in it)
'a right turn-off corporation's' is the wordplay.
'a right' becomes 'ar' (I've seen this in other clues).
'turn off' becomes 'bore' (I've seen this in another clue).
'corporation' becomes 'tum' ('corporation' can humorously mean belly fat).
'ar'+'bore'+'tum'='ARBORETUM'
(Other definitions for arboretum that I've seen before include "there's high growth" , "area with great growth" , "Botanic garden of trees" , "Garden where trees and shrubs are cultivated" , "Place for cultivation of trees" .)