Really bad senior cops strike back (6)
I believe the answer is:
dismal
'really bad' is the definition.
(dismal can mean bleak or grim)
'senior cops strike back' is the wordplay.
'senior cops' becomes 'DIs' (detective inspectors).
'strike' becomes 'lam' (to 'lam' means to hit hard).
'back' says the letters should be written in reverse.
'lam' backwards is 'mal'.
'dis'+'mal'='DISMAL'
(Other definitions for dismal that I've seen before include "Feeble" , "Sad, gloomy" , "Dull; doleful" , "describes economic science" , "dark" .)