Source of deadly green extract (4)
I believe the answer is:
draw
'extract' is the definition.
(I've seen this before)
'source of deadly green' is the wordplay.
'source of' suggests taking the first letters.
'green' becomes 'raw' ('raw' can be a synonym of 'green').
The first letter of 'deadly' is 'd'.
'd'+'raw'='DRAW'
(Other definitions for draw that I've seen before include "Tied score; sketch" , "2-2 in game, say" , "Drag" , "No-winner result" , "Sketch - tie" .)