Warning: a large branch by our cartoonist (5,4)
I believe the answer is:
Here is my best explanation:
'warning' is the definition.
'a large branch by our cartoonist' is the wordplay.
'a large' becomes 'l'.
'branch' becomes 'arm' ('arm' can be a synonym of 'branch').
'by' is a charade indicator (letters next to each other).
'our' becomes 'a' (I can't justify this - if you can you should believe this answer much more).
'cartoonist' becomes 'bell' (Steve Bell - famous UK cartoonist).
'larm' put after 'a' is 'alarm'.
(Other definitions for alarm bell that I've seen before include "It rings as a warning", "Ring it to signal danger", "Warning signal", "Danger signal", "Warning sound" and "Warning device".)