Cop turned killer in LA? (7)
I believe the answer is:
officer
'cop' is the definition.
(police officer)
'turned killer in la?' is the wordplay.
'turned' becomes 'off' (synonyms).
'killer in la?' becomes 'icer' (I can't explain this - if you can you should believe this answer much more).
'off'+'icer'='OFFICER'
Can you help me to learn more?
(Other definitions for officer that I've seen before include "He holds a commission" , "Higher-ranking soldier" , "Commissioned member of armed forces" , "served in the wardroom" , "Police constable" .)