King cutting an old soldier's head off (5)
I believe the answer is:
avert
'head off' is the definition.
(both can mean to prevent)
'king cutting an old soldier's' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'an' becomes 'a'.
'old soldier' becomes 'vet' (veteran).
'a'+'vet'='avet'
'r' put within 'avet' is 'AVERT'.
(Other definitions for avert that I've seen before include "Turn away, prevent" , "With prominent incisors" , "Ward off (bad event)" , "Turn away (ones eyes)" , "Forestall" .)