Promises royalty has regularly ignored (5)
I believe the answer is:
oaths
'promises' is the definition.
(I know this)
'royalty has regularly ignored' is the wordplay.
'regularly ignored' means one should take alternating letters (letters removed at regular intervals).
'royalty'+'has'='royaltyhas'
The alternating letters of 'royaltyhas' are 'OATHS'.
(Other definitions for oaths that I've seen before include "Swear words" , "Swearwords" , "Cursing" , "Solemn promises or curses" , "professions of loyalty" .)