Altogether a British duty to be dressed? (8,4)
I believe the answer is:
birthday suit
'altogether' is the definition.
('birthday suit' can be a synonym of 'altogether')
'a british duty to be dressed?' is the wordplay.
'to be dressed?' is an anagram indicator ('dress' can mean 'prepare').
'a'+'british'+'duty'='abritishduty'
'abritishduty' with letters rearranged gives 'BIRTHDAY SUIT'.
(Other definitions for birthday suit that I've seen before include "Invisible threads?" , "State of total nakedness" , "State of being unclothed" , "Invisible attire?" , "Invisible outfit?" .)