A willow. Very large? That’s right (5)
I believe the answer is:
osier
'a willow' is the definition.
(osier is a kind of willow)
'very large? that's right' is the wordplay.
'very large?' becomes 'os' (abbreviation for oversize).
'that's' becomes 'ie'.
'right' becomes 'r' (common abbreviation).
'os'+'ie'+'r'='OSIER'
(Other definitions for osier that I've seen before include "Willow growing shoots used in basketwork" , "Withy" , "Salix" , "Kind of twig used in basketwork" , "... source of twigs to make baskets" .)