Plant weapon base (9)
I believe the answer is:
arrowroot
'plant' is the definition.
(arrowroot is a kind of plant)
'weapon base' is the wordplay.
'weapon' becomes 'arrow' (arrow is a kind of weapon).
'base' becomes 'root' (synonyms).
'arrow'+'root'='ARROWROOT'
(Other definitions for arrowroot that I've seen before include "Medicinal plant" , "Starchy plant" , "that thickens" , "Nutritious starch from a West Indian plant" , "Fine cooking powder" .)