Stop Sartre somehow (6)
I believe the answer is:
arrest
'stop' is the definition.
(I know that stop can be written as arrest)
'sartre somehow' is the wordplay.
'somehow' indicates anagramming the letters.
'sartre' is an anagram of 'ARREST'.
(Other definitions for arrest that I've seen before include "Take into legal custody" , "apprehend a law-breaker" , "Run in" , "Stop - take into custody" , "Nab" .)