Republican in attendance with King's knightly order (6)
I believe the answer is:
garter
'knightly order' is the definition.
'republican in attendance with king's' is the wordplay.
'republican' becomes 'r' (abbreviation).
'in' indicates putting letters inside.
'attendance' becomes 'gate' (I've seen this before).
'with' means one lot of letters go next to another.
'king' becomes 'r' (r is abbreviation for rex).
'r' going inside 'gate' is 'garte'.
'garte'+'r'='GARTER'
(Other definitions for garter that I've seen before include "- Belt; - snake; - stitch" , "It holds a stocking up" , "Band holding up sock" , "Band to hold up a stocking" , "Support for a stocking" .)