A party with sailors below deck (5)
I believe the answer is:
adorn
'deck' is the definition.
(I've seen this in another clue)
'a party with sailors below' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'with' says to put letters next to each other.
'sailors' becomes 'RN' (abbreviation for Royal Navy).
'below' means one lot of letters go next to another (in a down clue, some letters go below others).
'a'+'do'+'rn'='ADORN'
(Other definitions for adorn that I've seen before include "Bedeck" , "Festoon" , "Deck" , "Beautify - decorate - enhance" , "Improve appearance" .)