Rosie cultivated willow (5)
I believe the answer is:
osier
'willow' is the definition.
(I know that osier is a type of willow)
'rosie cultivated' is the wordplay.
'cultivated' is an anagram indicator.
'rosie' with letters rearranged gives 'OSIER'.
(Other definitions for osier that I've seen before include "Basket-maker" , "Bit of tree" , "Basketry willow" , "one used to get weaving?" , "branch" .)