Mark in register police group reviewed (9)
I believe the answer is:
indicator
'mark' is the definition.
(thesaurus)
'in register police group reviewed' is the wordplay.
'register' becomes 'rota' (I've seen this in other clues).
'police group' becomes 'CID' (abbreviation for Criminal Investigation Department).
'reviewed' says the letters should be written in reverse.
'rota'+'cid'='rotacid'
'rotacid' back-to-front is 'dicator'.
'in'+'dicator'='INDICATOR'
(Other definitions for indicator that I've seen before include "Signal" , "This shows" , "shower" , "Pointer" , "This will show" .)