Trouble on Royal Navy deck (5)
I believe the answer is:
adorn
'deck' is the definition.
(I've seen this before)
'trouble on royal navy' is the wordplay.
'trouble' becomes 'ado' (both can mean bother or commotion).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'royal navy' becomes 'RN' (abbreviation for Royal Navy).
'ado'+'rn'='ADORN'
(Other definitions for adorn that I've seen before include "Attire" , "Beautify - decorate - enhance" , "Make more attractive" , "dress" , "get dressed!" .)