Battle against royal household (9)
I believe the answer is:
agincourt
'battle' is the definition.
(Agincourt is an example)
'against royal household' is the wordplay.
'against' becomes 'agin' (dialect form of against).
'royal household' becomes 'court'.
'agin'+'court'='AGINCOURT'
(Other definitions for agincourt that I've seen before include "1415 battle in the Pas-de-Calais" , "English-French battle" , "Henry V's battle" , "Battleground" , "battle site" .)