Like King Arthur's men, losing head regularly (7)
I believe the answer is:
nightly
'regularly' is the definition.
(happening every night)
'like king arthur's men losing head' is the wordplay.
'like king arthur's men' becomes 'knightly' (like the knights of the Round Table).
'losing head' means to remove the first letter.
'knightly' with its first letter taken away is 'NIGHTLY'.
(Other definitions for nightly that I've seen before include "when Casanova in action?" , "Happening every evening" , "Nocturnal" , "Regularly during the hours of darkness" , "Nocturnally" .)