Small willow (5)
I believe the answer is:
'small willow' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for osier that I've seen before include "may be a bit of a basket" , "Basket-making material" , "Willow for making baskets" , "Willow growing shoots used in basketwork" , "Bit of tree" .)