Old British hero housed six in blackout (8)
I believe the answer is:
oblivion
'blackout' is the definition.
(thesaurus)
'old british hero housed six' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'hero' becomes 'lion' (**).
'housed' means one lot of letters goes inside another.
'six' becomes 'vi' ('vi' can be a synonym of 'six').
'lion' enclosing 'vi' is 'livion'.
'o'+'b'+'livion'='OBLIVION'
'in' acts as a link.
(Other definitions for oblivion that I've seen before include "Neglected state" , "Total extinction" , "ignorance" , "State of total unconciousness" , "State of being mentally withdrawn or blank" .)