Whip restraining leader of riotous mob (6)
I believe the answer is:
throng
'mob' is the definition.
(I've seen this before)
'whip restraining leader of riotous' is the wordplay.
'whip' becomes 'thong' (I've seen this before).
'restraining' means one lot of letters goes inside another.
'leader of' suggests taking the first letters.
The initial letter of 'riotous' is 'r'.
'thong' enclosing 'r' is 'THRONG'.
(Other definitions for throng that I've seen before include "Milling crowd" , "Press" , "Large crowd or host" , "Large densely-packed crowd" , "Troop" .)