Mistake originally riling boss (6)
I believe the answer is:
gaffer
'boss' is the definition.
(gaffer can informally mean a boss)
'mistake originally riling' is the wordplay.
'mistake' becomes 'gaffe' (a gaffe is a mistake or error).
'originally' suggests taking the first letters.
The initial letter of 'riling' is 'r'.
'gaffe'+'r'='GAFFER'
(Other definitions for gaffer that I've seen before include "Boss, overseer" , "one prone to blunder socially?" , "Informally, a boss or foreman" , "one securing large fish" , "Informal boss or foreman" .)