### Clue: One word and all hell's let loose — they'll pay for this (5,4,4)
### Answer: HEADS WILL ROLL 
 ### Explanation: Definition: they'll pay for this
 **Anagram of**(… let loose) [ I(Roman numeral for "one") + WORD ***plus***(and) ALL HELL'S].
Defn: …, whoever's responsible.