Rosie cultivated a willow (5)
I believe the answer is:
osier
'a willow' is the definition.
(I know that osier is a type of willow)
'rosie cultivated' is the wordplay.
'cultivated' indicates an anagram.
'rosie' with letters rearranged gives 'OSIER'.
(Other definitions for osier that I've seen before include "Basket-maker" , "one used to get weaving?" , "Withy" , "Kind of willow used in basketwork" , "may be a bit of a basket" .)