Deck, where sailors joining a party (5)
I believe the answer is:
adorn
'deck' is the definition.
(thesaurus)
'sailors joining a party' is the wordplay.
'sailors' becomes 'rn' (abbreviation for Royal Navy).
'joining' is a charade indicator (letters next to each other).
'party' becomes 'do' (do is a kind of party).
'a'+'do'='ado'
'rn' after 'ado' is 'ADORN'.
'where' acts as a link.
(Other definitions for adorn that I've seen before include "Deck" , "Festoon" , "Embellish, decorate" , "Make more attractive" , "Attire" .)