Unacceptable in south (3)
I believe the answer is:
out
'unacceptable' is the definition.
(out can mean incorrect or inappropriate)
'in south' is the wordplay.
'in' indicates a hidden word.
'OUT' is hidden within 'south'.
(Other definitions for out that I've seen before include "jury considering verdict" , "Walking back to the pavilion" , "Unacceptable" , "Possibly caught" , "On strike - dismissed - unconsciousout" .)