Reason the Spanish will produce unwanted plant (9)
I believe the answer is:
groundsel
'unwanted plant' is the definition.
(type of weed)
'reason the spanish' is the wordplay.
'reason' becomes 'grounds' (grounds or justification for a particular action).
'the spanish' becomes 'el' ('the' in Spanish).
'grounds'+'el'='GROUNDSEL'
'will produce' acts as a link.
(Other definitions for groundsel that I've seen before include "Common weed" , "plant, unwanted?" , "feed to the birds" , "Plant of daisy family" .)