Riot queller (4)
I believe the answer is:
'riot queller' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mace that I've seen before include "Ceremonial staff or kind of spice" , "Tear gas - spice" , "Spice; ceremonial baton" , "Staff of office and a spice" , "Spice; heavy club" .)