Willow, very large, that is by river (5)
I believe the answer is:
osier
'willow' is the definition.
(I know that osier is a type of willow)
'large that is by river' is the wordplay.
'large' becomes 'os' (abbreviation for oversize).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'by' says to put letters next to each other.
'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 "Willow for making baskets" , "Willow with twigs for basket-weaving" , "may be a bit of a basket" , "one used to get weaving?" , "Small willow; its shoot" .)