A king ranks weapons (6)
I believe the answer is:
arrows
'weapons' is the definition.
(arrow is a kind of weapon)
'a king ranks' is the wordplay.
'a king' becomes 'ar' (I've seen this in other clues).
'ranks' becomes 'rows' (I've seen this before).
'ar'+'rows'='ARROWS'
(Other definitions for arrows that I've seen before include "The archer's missiles" , "Direction indicators" , "Flights on these" , "Bolts that are shot" , "Missiles for the archer" .)