South London police going north to make arrest (4)
I believe the answer is:
stem
'arrest' is the definition.
(both can mean to stop)
'south london police going north' is the wordplay.
'south' becomes 'S' (synonyms).
'london police' becomes 'Met' (London Metropolitan Police).
'going north' is a reversal indicator.
'met' reversed gives 'tem'.
's'+'tem'='STEM'
'to make' is the link.
(Other definitions for stem that I've seen before include "Ski turn and plant stalk" , "Tube" , "section of pipe" , "Basic skiing turn" , "Halt flow" .)