Taking over gangs, turn informer (10)
I believe the answer is:
'taking' is the definition.
The definition and answer can be both to do with ownership as well as being present participle verbs.
Perhaps there's an association between them I don't understand?
'gangs turn informer' is the wordplay.
'gangs' becomes 'posses' (I can't justify this - if you can you should give a lot more credence to this answer).
'turn informer' becomes 'sing' (I've seen this before).
'over' is the link.
Can you help me to learn more?
(Other definitions for possessing that I've seen before include "Owning" , "Taking control of" .)