Blow that is by king's sword (6)
I believe the answer is:
rapier
'sword' is the definition.
(I know this)
'blow that is by king's' is the wordplay.
'blow' becomes 'rap' (rap is a kind of blow).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'by' is a charade indicator (letters next to each other).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'rap'+'ie'+'r'='RAPIER'
(Other definitions for rapier that I've seen before include "Thrusting weapon" , "Straight sword with narrow blade" , "Thin sharp sword" , "Long slender sword" , "Fine-bladed sword" .)