Judges applying seriously exacting sanctions initially to leaderless hordes (8)
I believe the answer is:
assesses
'judges' is the definition.
(assess can mean to evaluate or judge)
'seriously exacting sanctions initially to leaderless hordes' is the wordplay.
'initially' says to take the initial letters.
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'leaderless' means to remove the first letter.
'hordes' becomes 'masses' (a horde is a mass of people).
The first letters of 'seriously exacting sanctions' is 'ses'.
'masses' with its initial letter taken off is 'asses'.
'ses' after 'asses' is 'ASSESSES'.
'applying' is the link.
(Other definitions for assesses that I've seen before include "Judges the worth of" , "Carries out tests" .)