Energy of mob riot (4)
I believe the answer is:
brio
'energy' is the definition.
(brio is a musical term meaning energy)
'of mob riot' is the wordplay.
'of' says the answer is hidden in the clue.
'BRIO' is hidden in the letters of 'mob riot'.
(Other definitions for brio that I've seen before include "Drive" , "Dash or verve, from Italian" , "Enthusiasm" , "(Of music) spirit" , "Vivacity of style" .)