Head supporting sustainable veg (5,4)
I believe the answer is:
green bean
'veg' is the definition.
'head supporting sustainable' is the wordplay.
'head' becomes 'bean' (bean is a kind of head**).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'sustainable' becomes 'green'.
'bean' put after 'green' is 'GREEN BEAN'.
(Other definitions for green bean that I've seen before include "runner?" , "French" , "Plant" , "Vegetable" .)