Once changed shape (4)
I believe the answer is:
cone
'shape' is the definition.
(cone shape is a kind of shape)
'once changed' is the wordplay.
'changed' is an anagram indicator.
'once' anagrammed gives 'CONE'.
(Other definitions for cone that I've seen before include "It produces the seed for the pine tree" , "may obstruct lane" , "Hollow object associated with ice cream and motorways" , "Volcanic peak" , "Solid figure" .)