Fear brawl must involve leader of rabble (6)
I believe the answer is:
fright
'fear' is the definition.
('fright' can be a synonym of 'fear')
'brawl must involve leader of rabble' is the wordplay.
'brawl' becomes 'fight' (I've seen this before).
'must involve' indicates putting letters inside.
'leader of' says to take the initial letters.
The first letter of 'rabble' is 'r'.
'fight' enclosing 'r' is 'FRIGHT'.
(Other definitions for fright that I've seen before include "Horror" , "Grotesque" , "Start" , "Sudden feeling of fear" , "Shock, sudden alarm" .)