Studies report on grasses (5)
I believe the answer is:
reeds
'grasses' is the definition.
(type of grass plant)
'studies report' is the wordplay.
'studies' becomes 'reads' ('read' can be a synonym of 'study').
'report' shows a homophone (sound like) (as heard, or reported by someone).
'reads' sounds like 'REEDS'.
'on' acts as a link.
(Other definitions for reeds that I've seen before include "River grasses" , "Tall water grasses on lake shore" , "Thatching stalks" , "Stops" , "Plants providing thatching material" .)