Mistake about an exile (6)
I believe the answer is:
banish
'exile' is the definition.
(to banish is to exile or cast out)
'mistake about an' is the wordplay.
'mistake' becomes 'bish' (informal term for an error).
'about' means one lot of letters goes inside another.
'bish' going around 'an' is 'BANISH'.
(Other definitions for banish that I've seen before include "Send (someone) away" , "Condemn to exile" , "Send into exile" , "Send away (as punishment)" , "Expel from a community or group" .)