Wise men in from Agincourt (4)
I believe the answer is:
magi
'wise men' is the definition.
(the three wise men from the nativity)
'in from agincourt' is the wordplay.
'in' indicates the answer is hidden within the clue.
'MAGI' can be found hidden inside 'from agincourt'.
(Other definitions for magi that I've seen before include "The sages who visited Jesus and Mary and Joseph shortly after Jesus was born" , "The Three Kings" , "Priests" , "Star witnesses" , "Eastern men of lore; Epiphany figures" .)