Boss almost making mistake (5)
I believe the answer is:
gaffe
'mistake' is the definition.
(a gaffe is a mistake or error)
'boss almost' is the wordplay.
'boss' becomes 'gaffer' (gaffer can informally mean a boss).
'almost' means to remove the last letter (most of the word but not all of it).
'gaffer' with its final letter taken off is 'GAFFE'.
'making' acts as a link.
(Other definitions for gaffe that I've seen before include "Clanger" , "A blunder, faux pas" , "A foolish error" , "embarrassing mistake" , "bad mistake" .)