To attack king, assistance is required (4)
I believe the answer is:
raid
'to attack' is the definition.
(raiding is a kind of attacking)
'king assistance is required' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex**).
'assistance' becomes 'aid' ('aid' can be a synonym of 'assistance').
'is required' says to put letters next to each other.
'r'+'aid'='RAID'
(Other definitions for raid that I've seen before include "Invasion" , "Sudden swift attack" , "Surprise police visit" , "Smash-and-grab?" , "military operation" .)