Start to observe criminal holding revolver over a figure (7)
I believe the answer is:
octagon
'a figure' is the definition.
(octagon is a kind of figure)
'start to observe criminal holding revolver over' is the wordplay.
'start to' indicates taking the first letters.
'holding revolver over' becomes 'tagon' (I can't justify this - if you can you should give a lot more credence to this answer).
The first letters of 'observe criminal' is 'oc'.
'oc'+'tagon'='OCTAGON'
Can you help me to learn more?
(Other definitions for octagon that I've seen before include "regular form" , "Eight-sided figure" , "plane figure" , "Eight-sided shape" , "Figure with 8 sides" .)