Different alert sign shapes (9)
I believe the answer is:
triangles
'shapes' is the definition.
(triangle is a kind of shape)
'different alert sign' is the wordplay.
'different' indicates an anagram (letters placed in a different order).
'alert'+'sign'='alertsign'
'alertsign' with letters rearranged gives 'TRIANGLES'.
(Other definitions for triangles that I've seen before include "Three-sided figures" , "Three-sided polygons" , "Geometrical figures" , "Percussion instruments" , "Three-sided shapes" .)