Stop somewhat rude sister (6)
I believe the answer is:
desist
'stop' is the definition.
(thesaurus)
'somewhat rude sister' is the wordplay.
'somewhat' indicates the answer is hidden within the clue.
'DESIST' is hidden within 'rude sister'.
(Other definitions for desist that I've seen before include "'Give over, refrain (6)'" , "Stop, don't continue" , "Stop (doing something)" , "Cease (from doing something)" , "Abstain, refrain" .)