One or the other revealed by Keith erroneously (6)
I believe the answer is:
either
'one or the other' is the definition.
(I've seen this before)
'revealed by keith erroneously' is the wordplay.
'revealed by' says the answer is hidden in the clue.
'EITHER' is hidden within 'keith erroneously'.
(Other definitions for either that I've seen before include "Each of two; moreover" , "Her tie (anag.)" , "Word introducing first of two 16s" , "One of two alternatives" , "Word indicating the following alternatives" .)