Police ultimately detain grass (4)
I believe the answer is:
lawn
'grass' is the definition.
(a lawn is an area of grass)
'police ultimately detain' is the wordplay.
'police' becomes 'law' (synonyms).
'ultimately' says to take the final letters.
The final letter of 'detain' is 'n'.
'law'+'n'='LAWN'
(Other definitions for lawn that I've seen before include "Grassy patch" , "Area of mown grass" , "Green area" , "sleeves composer?" , "Bowling green" .)