Trouble with British sailors below deck (5)
I believe the answer is:
adorn
'deck' is the definition.
(I've seen this before)
'trouble with british sailors below' is the wordplay.
'trouble with' becomes 'ado' (both can mean bother or commotion. I am not sure about the 'with' bit.).
'british sailors' becomes 'RN' (Royal Navy).
'below' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'ado'+'rn'='ADORN'
Can you help me to learn more?
(Other definitions for adorn that I've seen before include "Beautify - decorate - enhance" , "Make more attractive" , "get dressed!" , "dress" , "Festoon" .)