Deserter from company on the outskirts of Agincourt, supporting revolution (8)
I believe the answer is:
turncoat
'deserter' is the definition.
(I know that deserter can be written as turncoat)
'company on the outskirts of agincourt supporting revolution' is the wordplay.
'company' becomes 'co' (abbreviation).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'the outskirts of' says to hollow out the word (remove centre letters) (letters on the edges or outskirts are used).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'revolution' becomes 'turn' (revolution can mean a turn or rotation).
'agincourt' with its middle removed is 'at'.
'co' put next to 'at' is 'coat'.
'coat' after 'turn' is 'TURNCOAT'.
'from' acts as a link.
(Other definitions for turncoat that I've seen before include "'Renegade, traitor (8)'" , "Traitor with inverted garment" , "Traitor, defector" , "One betraying party" , "Traitor with reversed garment" .)