Plant weapon on source (9)
I believe the answer is:
arrowroot
'plant' is the definition.
(arrowroot is a kind of plant)
'weapon on source' is the wordplay.
'weapon' becomes 'arrow' (arrow is a kind of weapon).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'source' becomes 'root' (both can mean the origin of something).
'arrow'+'root'='ARROWROOT'
(Other definitions for arrowroot that I've seen before include "culinary powder" , "Fine cooking powder" , "that thickens" , "Medicinal plant" , "Starchy plant" .)