One may upset royal servant (6)
I believe the answer is:
yeoman
'royal servant' is the definition.
(I've seen this before)
'one may upset' is the wordplay.
'upset' indicates an anagram.
'one'+'may'='onemay'
'onemay' anagrammed gives 'YEOMAN'.
(Other definitions for yeoman that I've seen before include "16th-century freeholder of a small farm" , "Officer in the bodyguard of the British monarch" , ". . . . . . of the Guard (operetta) (6)" , "... of the Guard (Gilbert and Sullivan)" , "Former royal servant" .)