Rosie is in willowy shape (5)
I believe the answer is:
osier
'in willowy shape' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'rosie' is the wordplay.
'rosie' can be anagrammed to 'OSIER'.
Though, I am unsure how the anagram is indicated.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for osier that I've seen before include "Tree - Rosie (anag)" , "Basket-maker's willow" , "Basket material" , "may be a bit of a basket" , "Small willow; its shoot" .)