Ace party on Royal Navy's decks (6)
I believe the answer is:
adorns
'decks' is the definition.
(I've seen this in another clue)
'ace party on royal navy's' is the wordplay.
'ace' becomes 'a' (abbreviation).
'party' becomes 'do' (do is a kind of party).
'on' means one lot of letters go next to another.
'royal navy's' becomes 'RN's' (abbreviation for Royal Navy).
'a'+'do'+'rns'='ADORNS'
(Other definitions for adorns that I've seen before include "'Decorates, embellishes (6)'" , "Adds ornamentation" , "Bedecks" , "Decorates with ornaments" , "decks" .)