As a stop sign can go a lot out of shape (9)
I believe the answer is:
octagonal
'as a stop sign' is the definition.
I can't judge whether this defines the answer.
'can go a lot out of shape' is the wordplay.
'out of shape' is an anagram indicator.
'can'+'go'+'a'+'lot'='cangoalot'
'cangoalot' is an anagram of 'OCTAGONAL'.
Can you help me to learn more?
(Other definitions for octagonal that I've seen before include "Eight sided" , "In unusual shape" , "Eight-sided" , "With eight sides and angles" , "With many sides" .)