Slow down with Sartre novel (6)
I believe the answer is:
arrest
'slow down' is the definition.
'sartre novel' is the wordplay.
'novel' is an anagram indicator.
'sartre' is an anagram of 'ARREST'.
'with' is the link.
(Other definitions for arrest that I've seen before include "apprehend a law-breaker" , "Seize (a crime suspect)" , "Seize and take into custody" , "Stop - detain" , "Stop (on suspicion)" .)