Branch of thieves' organisation tried reforming (8)
I believe the answer is:
dendrite
'branch of thieves'' is the definition.
I can't judge whether this definition defines the answer.
'organisation tried reforming' is the wordplay.
'organisation' becomes 'den' (I can't justify this - if you can you should believe this answer much more).
'reforming' indicates an anagram.
'tried' with letters rearranged gives 'drite'.
'den'+'drite'='DENDRITE'
Can you help me to learn more?
(Other definitions for dendrite that I've seen before include "Dried net of the nerve fibre" , "Crystalline tree-like structure" .)