Figure soon chaired assembly (11)
I believe the answer is:
icosahedron
'figure' is the definition.
(I've seen this before)
'soon chaired assembly' is the wordplay.
'assembly' indicates an anagram.
'soon'+'chaired'='soonchaired'
'soonchaired' with letters rearranged gives 'ICOSAHEDRON'.
(Other definitions for icosahedron that I've seen before include "Twenty-faced solid" , "Figure" .)