Jack in part of Sevastopol (4)
I believe the answer is:
stop
'jack in' is the definition.
'part of sevastopol' is the wordplay.
'part of' indicates a hidden word.
'STOP' can be found hidden inside 'sevastopol'.
(Other definitions for stop that I've seen before include "Red light signal" , "Go no further" , "Plug - organ knob - bus halt - period" , "Come to an end" , "Withhold large amount raised" .)