Tall riverside grasses (5)
I believe the answer is:
reeds
'tall riverside grasses' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for reeds that I've seen before include "Music-makers" , "Tall water grasses on lake shore" , "They vibrate to produce the sound of various musical instruments" , "Thin pieces of cane in musical instruments" , "Thatching stalks" .)