Mix of dried flowers and grass -- chuck it down drain, periodically (9)
I believe the answer is:
potpourri
'mix of dried flowers' is the definition.
'potpourri' can be an answer for 'mix' (thesaurus). I am not certain of the 'of dried flowers' bit.
'grass chuck it down drain periodically' is the wordplay.
'grass' becomes 'pot' (cannabis).
'chuck it down' becomes 'pour' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'periodically' means one should take alternating letters (letters must be selected 'every so often').
The alternate letters of 'drain' are 'ri'.
'pot'+'pour'+'ri'='POTPOURRI'
'and' is the link.
Can you help me to learn more?
(Other definitions for potpourri that I've seen before include "Scented petals and spices" , "Perfumed mixture; medley" , "stew" , "collection with pleasant bouquet" , "Mixture of petals, spices etc." .)