Bill ducks a subject to be ducked (5)
I believe the answer is:
taboo
'a subject to be ducked' is the definition.
'bill ducks' is the wordplay.
'bill' becomes 'tab' (tab is a kind of bill).
'ducks' becomes 'oo' (I've seen this in other clues).
'tab'+'oo'='TABOO'
(Other definitions for taboo that I've seen before include "Ritual prohibition" , "Not permitted" , "Not to be touched or used, banned for religious reasons" , "unacceptable" , "A ban or prohibition of superstitious kind" .)