Tree, large, that is beside river (5)
I believe the answer is:
osier
'tree' is the definition.
(osier is a kind of tree)
'large that is beside river' is the wordplay.
'large' becomes 'os' (abbreviation for oversize).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'beside' is a charade indicator (letters next to each other) (I've seen this in other clues).
'river' becomes 'r' (abbreviation).
'os'+'ie'+'r'='OSIER'
(Other definitions for osier that I've seen before include "Weaving cane for Rosie" , "Willow for making baskets" , "Tree - Rosie (anag)" , "branch" , "Basket maker" .)