Oust police officer’s back-up group primarily serving sheriff (10)
I believe the answer is:
dispossess
'oust' is the definition.
(thesaurus)
'police officer's back-up group primarily serving sheriff' is the wordplay.
'police officer's' becomes 'DI's' (detective inspector).
'back up group' becomes 'posse' (I can't justify this - if you can you should give a lot more credence to this answer).
'primarily' indicates taking the first letters.
The initial letters of 'serving sheriff' is 'ss'.
'dis'+'posse'+'ss'='DISPOSSESS'
Can you help me to learn more?
(Other definitions for dispossess that I've seen before include "Expel" , "Deprive of land and property" , "Drive out l" , "Deprive (someone) of land/property" .)