The weapons are a knock out, sire! (7)
I believe the answer is:
Here is my best explanation:
'the weapons' is the definition.
(rapier is a kind of weapon)
'a knock out sire' is the subsidiary indication.
'a knock' becomes 'rap' ('rap' can be a synonym of 'knock').
'out' indicates an anagram.
'sire' anagrammed gives 'iers'.
'are' is the link.
(Other definitions for rapiers that I've seen before include "Double-edged swords" and "Straight swords with narrow blades".)