Part of the service dressing royal (6)
I believe the answer is:
saucer
'part of the service' is the definition.
(I've seen this before)
'dressing royal' is the wordplay.
'dressing' becomes 'sauce' (I've seen this before).
'royal' becomes 'r' (abbreviation).
'sauce'+'r'='SAUCER'
(Other definitions for saucer that I've seen before include "A small shallow curved dish" , "Piece of tableware, goes with cup" , "Small dish, goes with cup" , "less than a plate?" , "If flying, it would have alien pilots" .)