Police finally breaking in to discover addict (5)
I believe the answer is:
fiend
'addict' is the definition.
(I've seen this in another clue)
'police finally breaking in to discover' is the wordplay.
'finally' indicates one should take the final letters.
'breaking in' indicates putting letters inside.
'to discover' becomes 'find' (synonyms).
The final letter of 'police' is 'e'.
'e' put inside 'find' is 'FIEND'.
(Other definitions for fiend that I've seen before include "Operatic super-soprano" , "Demon, ogre" , "Demon; cruel person" , "Evil supernatural being" , "monster" .)