Tree, very large, that is by river (5)
I believe the answer is:
osier
'tree' is the definition.
(osier is a kind of tree)
'large that is by river' is the wordplay.
'large' becomes 'os' (abbreviation for oversize).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'by' means one lot of letters go next to another.
'river' becomes 'r' (abbreviation for river).
'os'+'ie'+'r'='OSIER'
'very' acts as a link.
(Other definitions for osier that I've seen before include "Basketry willow" , "Willow for basket making" , "Bit of tree" , "... source of twigs to make baskets" , "branch" .)