He's in error, you know (4)
I believe the answer is:
rory
'he's' is the definition.
'in error you know' is the wordplay.
'in' indicates a hidden word.
'RORY' is hidden within 'error you know'.
(Other definitions for rory that I've seen before include "Gaelic man?" , "Male" , "Irishman" , "Form of Gaelic name Ruairidh, rhymes with Conservative" , "Fellow" .)