Police taking on new grass (4)
I believe the answer is:
lawn
'grass' is the definition.
(thesaurus)
'police taking on new' is the wordplay.
'police' becomes 'law' ('law' can be a synonym of 'police').
'taking on' means one lot of letters go next to another.
'new' becomes 'n'.
'law'+'n'='LAWN'
(Other definitions for lawn that I've seen before include "material" , "Area of smooth grass - fine linen" , "Area of short grass" , "Blades trimmed" , "Stretch of grass" .)