Figure about to move into criminal haunt (7)
I believe the answer is:
decagon
'figure' is the definition.
(type of geometrical figure)
'about to move into criminal haunt' is the wordplay.
'about' becomes 'ca' (abbreviation for 'circa').
'to move' becomes 'go' (to go is to move).
'into' is an insertion indicator.
'criminal haunt' becomes 'den' (eg a den of thieves).
'ca'+'go'='cago'
'cago' placed into 'den' is 'DECAGON'.
(Other definitions for decagon that I've seen before include "Ten-sided plane figure" , "Plane shape" , "Go, dance on geometric figure with ten sides" , "10-sided figure" , "Ten-sided shape" .)