Decorate for service to a party (5)
I believe the answer is:
adorn
'decorate' is the definition.
(synonyms)
'service to a party' is the wordplay.
'service' becomes 'RN' (abbreviation for Royal Navy).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'party' becomes 'do' (do is a kind of party).
'a'+'do'='ado'
'rn' put after 'ado' is 'ADORN'.
'for' acts as a link.
(Other definitions for adorn that I've seen before include "get dressed!" , "Make more attractive" , "Embellish, decorate" , "Improve appearance" , "Attire" .)