Fight police officer in cell blocks (5)
I believe the answer is:
wards
'cell blocks' is the definition.
(a ward is a part of a prison)
'fight police officer' is the wordplay.
'fight' becomes 'war' (warring is a kind of fighting).
'police officer' becomes 'DS' (short for detective sergeant).
'war'+'ds'='WARDS'
'in' is the link.
(Other definitions for wards that I've seen before include "Hospital rooms" , "Charges" , "Hospital divisions" .)