...grass right by rising river (4)
I believe the answer is:
reed
'grass' is the definition.
(I've seen this before)
'right by rising river' is the wordplay.
'right' becomes 'r' (common abbreviation).
'by' says to put letters next to each other.
'rising' is a reversal indicator (going up in a down clue).
'river' becomes 'dee' (River Dee**).
'dee' reversed gives 'eed'.
'r'+'eed'='REED'
(Other definitions for reed that I've seen before include "Marsh or water plant" , "Tall water-loving grass" , "oboe part" , "Slender grass of wet areas" , "Clarinet part; plant" .)