Container used for river grass? (9)
I believe the answer is:
flowerpot
'container' is the definition.
(flowerpot is a kind of container)
'river grass?' is the wordplay.
'river' becomes 'flower' (a river flows).
'grass?' becomes 'pot' (cannabis).
'flower'+'pot'='FLOWERPOT'
'used for' acts as a link.
(Other definitions for flowerpot that I've seen before include "What may hold" , "Geranium holder?" , "aid for gardeners" , "Something grows in this" , "Place for growing" .)