Stop special police retreating (4)
I believe the answer is:
stem
'stop' is the definition.
(stemming is a kind of stopping)
'special police retreating' is the wordplay.
'special' becomes 's' (common abbreviation as in Special Air Service).
'police' becomes 'met' (I've seen this in other clues).
'retreating' is a reversal indicator.
'met' back-to-front is 'tem'.
's'+'tem'='STEM'
(Other definitions for stem that I've seen before include "Staunch (flow)" , "Stalk; restrict (a flow)" , "Thwart; support" , "feature of wine glass" , "something growing?" .)