Deck that is trouble for the Royal Navy (5)
I believe the answer is:
adorn
'deck' is the definition.
(I've seen this before)
'trouble for the royal navy' is the wordplay.
'trouble' becomes 'ado' (both can mean bother or commotion).
'for' means one lot of letters go next to another.
'the royal navy' becomes 'RN' (abbreviation for Royal Navy).
'ado'+'rn'='ADORN'
'that is' is the link.
(Other definitions for adorn that I've seen before include "get dressed!" , "Attire" , "Deck" , "Improve appearance" , "Beautify - decorate - enhance" .)