A bar erected on quarter deck (5)
I believe the answer is:
adorn
'deck' is the definition.
(I've seen this in another clue)
'a bar erected on quarter' is the wordplay.
'bar' becomes 'rod' (I've seen this before).
'erected' says the letters should be written in reverse (in a down clue, letters go up).
'on' means one lot of letters go next to another.
'quarter' becomes 'n'.
'rod' written backwards gives 'dor'.
'a'+'dor'+'n'='ADORN'
(Other definitions for adorn that I've seen before include "dress" , "Embellish, decorate" , "Beautify - decorate - enhance" , "Deck" , "Bedeck" .)