Unacceptable in Southampton (3)
I believe the answer is:
out
'unacceptable' is the definition.
(out can mean incorrect or inappropriate)
'in southampton' is the wordplay.
'in' indicates a hidden word.
'OUT' can be found hidden inside 'southampton'.
(Other definitions for out that I've seen before include "Dismissed, in cricket" , "Away from home" , "Not indoors" , "(Of a flower) open" , "SECOND PART OF 16" .)