Arrest a skinhead, at best (4)
I believe the answer is:
stop
'arrest' is the definition.
('stop' can be a synonym of 'arrest')
'a skinhead at best' is the wordplay.
'a skinhead' becomes 's' (head letter of 'skin').
'at' says to put letters next to each other.
'best' becomes 'top' ('top' can be similar in meaning to 'best').
's'+'top'='STOP'
(Other definitions for stop that I've seen before include "Blockage" , "Wind up" , "Come to a halt" , "high card in bridge" , "Organ knob; get bus here" .)