Potentially hostile agents work to break secret police chief (8)
I believe the answer is:
bacteria
'potentially hostile agents' is the definition.
'bacteria' can be an answer for 'agents' (I have seen 'Agents of infection ' mean 'bacteria' so perhaps 'agents' could also mean 'bacteria'). I am not sure about the 'potentially hostile' bit.
'work to break secret police chief' is the wordplay.
'work' becomes 'act' (synonyms).
'to break' indicates putting letters inside.
'secret police chief' becomes 'beria' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'act' going inside 'beria' is 'BACTERIA'.
Can you help me to learn more?
(Other definitions for bacteria that I've seen before include "Care a bit about sources of diseases" , "Agents of infection" , "I eat crab (anag)" , "Single-celled microorganisms" , "Tiny causes of disease" .)