Weapon left in the possession of Deputy Führer unlikely to cause any damage (8)
I believe the answer is:
'deputy fuhrer unlikely to cause any damage' is the definition.
I can't judge whether this definition defines the answer.
'weapon left in the possession' is the wordplay.
'weapon' becomes 'arm' (synonyms).
'left' becomes 'L' (common abbreviation).
'in the' is an insertion indicator.
'possession' becomes 'hess' (I can't justify this - if you can you should give a lot more credence to this answer).
'arml' placed within 'hess' is 'HARMLESS'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for harmless that I've seen before include "Benign, innocuous" , "Not injurious" , "Safe; innocent" , "Completely safe" , "Causing no damage" .)