One word and all hell's let loose — they'll pay for this (5,4,4)
I believe the answer is:
heads will roll
'they'll pay for this' is the definition.
(phrase meaning people will be punished)
'one word and all hell's let loose' is the wordplay.
'one' becomes 'i' (Roman numeral).
'and' is a charade indicator (letters next to each other).
'let loose' is an anagram indicator.
'i'+'word'+'all'+'hells'='iwordallhells'
'iwordallhells' is an anagram of 'HEADS WILL ROLL'.
(Other definitions for heads will roll that I've seen before include "Someone will have to pay for this!" , ""Some of you will be fired for this!"" , "People will be dismissed" , "People are going to carry the can" , "People are going to pay for this!" .)