Alerting criminal, figure that! (8)
I believe the answer is:
triangle
'figure that' is the definition.
'triangle' can be an answer for 'figure' (triangle is a kind of figure). I am not certain of the remainder of the definition.
'alerting criminal' is the wordplay.
'criminal' indicates anagramming the letters.
'alerting' is an anagram of 'TRIANGLE'.
Can you help me to learn more?
(Other definitions for triangle that I've seen before include "diamond cut in half?" , "Polygon, could be isosceles" , "Geometric shape (eternal, maybe)" , "complex romantic situation?" , "Geometrical figure - musical instrument" .)