Expert, long-serving royal polisher (6)
I believe the answer is:
buffer
'polisher' is the definition.
(synonyms)
'expert long-serving royal' is the wordplay.
'expert' becomes 'buff' (buff can mean an expert or aficionado).
'long-serving royal' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth II).
'buff'+'er'='BUFFER'
(Other definitions for buffer that I've seen before include "Shock absorber, it lessens impact" , "Silly old man" , "Shock absorber - ineffectual fellow" , "doddery old man" , "Thing or structure that lessens effect of impact" .)