Former police upset reserves in cover (7)
I believe the answer is:
curtain
'cover' is the definition.
(thesaurus)
'former police upset reserves in' is the wordplay.
'former police' becomes 'ruc' (Royal Ulster Constabulary).
'upset' is a reversal indicator.
'reserves' becomes 'ta' (I've seen this in other clues).
'ruc' back-to-front is 'cur'.
'cur'+'ta'+'in'='CURTAIN'
(Other definitions for curtain that I've seen before include "Hanging material screen" , "Stage concealer" , "Window material" , "Final instruction of play" , "Window drape" .)