Rosie's willowy form (5)

I believe the answer is:

osier

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'willowy form' is the definition.
(osier is a type of willow tree)

'rosie's' is the wordplay.
'rosie' anagrams to 'OSIER'.
However, I am not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for osier that I've seen before include "Tree for basket making" , "Willow for basket-making" , "one used to get weaving?" , "Bit of tree" , "Willow for making baskets" .)

I've seen this clue in the Evening Standard, The Telegraph, The Guardian and The Globe and Mail.

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