Encourage scheme with square vegetable (8)
I believe the answer is:
eggplant
'vegetable' is the definition.
(I've seen this before)
'encourage scheme with square' is the wordplay.
'encourage' becomes 'egg'.
'scheme' becomes 'plan' (I've seen this before).
'with' says to put letters next to each other.
'square' becomes 't' (a T-Square is a technical drawing instrument).
'egg'+'plan'+'t'='EGGPLANT'
(Other definitions for eggplant that I've seen before include "What goes into moussaka" , "Another word for aubergine" , "Its fruit is aubergine" , "Vegetable" , "US name for aubergine" .)