A Mexican agave plant cultivated for its large fleshy leaves, which yield a stiff fibre used for making rope (5)

(Other definitions for sisal that I've seen before include "Agave rope-fibre" , "Rope-fibre made from leaves of tropical plant" , "material for painter, say?" , "Sails (anag) - fibre" , "Rope fibre from leaves of tropical plant" .)