Go on - act out this figure (7)
I believe the answer is:
octagon
'this figure' is the definition.
(type of geometrical figure)
'go on act out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'go'+'on'+'act'='goonact'
'goonact' with letters rearranged gives 'OCTAGON'.
(Other definitions for octagon that I've seen before include "regular form" , "plane figure" , "Shape of a square with the corners cut off" , "Eight-sided room" , "Figure with 8 sides" .)