Perhaps retreat in blunder, losing knight and corps entirely (5,4)

'perhaps retreat' is the definition.
(I know that retreat is a type of bugle call)

'blunder losing knight and corps entirely' is the wordplay.
'blunder' becomes 'bungle' (both can mean a mistake).
'losing' indicates named letters should be taken away.
'knight' becomes 'n' (chess abbreviation).
'and' is a charade indicator (letters next to each other).
'corps entirely' becomes 'call' (I can't justify this - if you can you should believe this answer much more).
'bungle' with 'n' taken away is 'bugle'.
'bugle'+'call'='BUGLE-CALL'

'in' is the link.