Charlie, turn key to go outside! Mug! (7)
I believe the answer is:
assault
'mug' is the definition.
(mug can mean to attack and rob someone)
'charlie turn key to go outside' is the wordplay.
'charlie' becomes 'ass' (both can mean a foolish person).
'turn' becomes 'U' (a 'U-turn' is a kind of turn).
'key' becomes 'alt' (the Alt keyboard button).
'to go outside' is an insertion indicator.
'u' going inside 'alt' is 'ault'.
'ass'+'ault'='ASSAULT'
(Other definitions for assault that I've seen before include "Storm" , "offensive" , "Sudden violent attack" , "The fool starts the attack" , "Onslaught" .)