Sediment found by the Mexican common weed (9)
I believe the answer is:
groundsel
'common weed' is the definition.
(I've seen this before)
'sediment found by the mexican' is the wordplay.
'sediment' becomes 'grounds' (as in coffee grounds).
'found by' is a charade indicator (letters next to each other).
'the mexican' becomes 'el' ('the' in Spanish).
'grounds'+'el'='GROUNDSEL'
(Other definitions for groundsel that I've seen before include "Common weed" , "Plant of daisy family" , "feed to the birds" , "plant, unwanted?" .)