Free up king's ceremonial weapon (4)
I believe the answer is:
dirk
'ceremonial weapon' is the definition.
(kind of dagger)
'free up king's' is the wordplay.
'free' becomes 'rid' (similar in meaning).
'up' is a reversal indicator (in down clue: letters go upwards).
'king' becomes 'K' (abbreviation for king in chess).
'rid' written backwards gives 'dir'.
'dir'+'k'='DIRK'
(Other definitions for dirk that I've seen before include "Scottish Highlander's dagger" , "Scots dagger" , "Daggert" , "Dagger with a long straight blade" , "Highland dagger" .)