Arrest unruly French existentialist (6)
I believe the answer is:
sartre
'french existentialist' is the definition.
(I've seen this in another clue)
'arrest unruly' is the wordplay.
'unruly' is an anagram indicator.
'arrest' is an anagram of 'SARTRE'.
(Other definitions for sartre that I've seen before include "French philosopher, 1905-80" , "French existentialist philosopher and writer" , "Jean-Paul ___, French existentialist, d. 1980" , "Sooner" , "Jean-Paul -, French philosopher/author" .)