A Beefeater, say, leading king's attendants in battle (9)
I believe the answer is:
agincourt
'battle' is the definition.
(Agincourt is an example)
'a beefeater say leading king's attendants' is the wordplay.
I cannot quite understand how this works, but
'a' is within the answer.
'leading' could be 'in' (in government) and 'in' is located in the answer.
'king' could be 'r' (abbreviation of Latin rex) and 'r' is located in the answer.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for agincourt that I've seen before include "French village" , "English-French battle" , "1415 battle in the Pas-de-Calais" , "Battleground" , "Battle in Hundred Years' War" .)