Like Arthur’s men, disowning king at regular intervals (7)
I believe the answer is:
nightly
'at regular intervals' is the definition.
(happening every night)
'like arthur's men disowning king' is the wordplay.
'like arthur's men' becomes 'knightly' (like King Arthur's knights of the Round Table).
'disowning' is a deletion indicator.
'king' becomes 'K' (abbreviation for king in chess).
'knightly' with 'k' removed is 'NIGHTLY'.
(Other definitions for nightly that I've seen before include "Happening every evening" , "when Casanova in action?" , "Every day between sunset and sunrise" , "At the end of each day" , "Nocturnal" .)