Castigating the informer with being outside (8)
I believe the answer is:
berating
'castigating' is the definition.
The answer and definition can be both to do with communicating as well as being present participle verbs.
Perhaps they are linked in a way I don't understand?
'informer with being outside' is the wordplay.
'informer with' becomes 'rat' (I can't justify this - if you can you should believe this answer much more).
'outside' is an insertion indicator.
'rat' put within 'being' is 'BERATING'.
'the' is the link.
Can you help me to learn more?
(Other definitions for berating that I've seen before include "Reprimanding" , "Taking to task" , "Rebuking loudly" , "carpeting" , "Rebuking severely" .)