Reportedly study grass (4)
I believe the answer is:
reed
'grass' is the definition.
(I've seen this before)
'reportedly study' is the wordplay.
'reportedly' shows a homophone (sound like).
'study' becomes 'read' (synonyms).
'read' sounds like 'REED'.
(Other definitions for reed that I've seen before include "Tall grass that grows in swamps" , "Riverside grass" , "It grows in water or marshy land" , "Slender grass of wet areas" , "Marsh or water plant" .)