Charlie's terror left during attack (9)
I believe the answer is:
simpleton
'charlie's' is the definition.
'terror left during attack' is the wordplay.
'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'**).
'imp'+'l'='impl'
'impl' going inside 'seton' is 'SIMPLETON'.
(Other definitions for simpleton that I've seen before include "Foolish or gullible person" , "Ass" , "Idiot" , "Dope" , "Unintelligent person" .)