Usual thing — article describing conquest (6)
I believe the answer is:
norman
'conquest' is the definition.
(Norman Conquest is an example)
'usual thing article' is the wordplay.
'usual thing' becomes 'norm' (normal conditions).
'article' becomes 'an' (indefinite article).
'norm'+'an'='NORMAN'
'describing' acts as a link.
(Other definitions for norman that I've seen before include "Someone who's not British" , "Romanesque architectural style" , "Black Wednesday's Chancellor of the Exchequer" , "British comedian, once a cult figure in Albania, d. 2010" , "Australian golfer" .)