Blunder made by manager losing final piece (5)
I believe the answer is:
gaffe
'blunder' is the definition.
(gaffe is a kind of blunder)
'manager losing final piece' is the wordplay.
'manager' becomes 'gaffer' (gaffer can mean a manager or boss).
'losing final piece' means to remove the last letter.
'gaffer' with its final letter taken off is 'GAFFE'.
'made by' acts as a link.
(Other definitions for gaffe that I've seen before include "Indiscretion" , "slip" , "indiscreet remark" , "Boob" , "A foolish error" .)