Charlie's terror left during attack (9)
I believe the answer is:
Here is my best explanation:
'charlie's' is the definition.
'terror left during attack' is the subsidiary indication.
'terror' becomes 'imp'.
'left' becomes 'l' (common abbreviation).
'during' means one lot of letters goes inside another.
'attack' becomes 'seton' ('set on' can be a synonym of 'attack'**).
'impl' inserted within 'seton' is 'simpleton'.
(Other definitions for simpleton that I've seen before include "Natural", "Naive person", "Foolish or gullible person", "Dunce", "Dope", "Blockhead", "Fool", "Unintelligent person", "Nincompoop" and "Gullible type".)