In horrid Acton, go figure (7)
I believe the answer is:
octagon
'figure' is the definition.
(type of geometrical figure)
'in horrid acton go' is the wordplay.
'in' indicates putting letters inside.
'horrid' is an anagram indicator.
'acton' anagrammed gives 'octan'.
'octan' enclosing 'go' is 'OCTAGON'.
(Other definitions for octagon that I've seen before include "Eight-sided figure" , "regular form" , "with eight angles" , "Figure of many sides" , "Eight-sided room" .)