Boss is right by mistake! (6)
I believe the answer is:
gaffer
'boss' is the definition.
(I know that boss can be written as gaffer)
'right by mistake' is the wordplay.
I cannot really see how this works, but
'right' could be 'r' (common abbreviation) and 'r' is present in the answer.
'mistake' could be 'gaffe' (gaffe is a kind of mistake) and 'gaffe' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
(Other definitions for gaffer that I've seen before include "Foreperson of a work squad" , "blunderer" , "Informal boss or foreman" , "Chief electrician on a film set" , "one securing large fish" .)