Take a piece from organisation with a return address (5)
I believe the answer is:
unarm
'take a piece from' is the definition.
(I've seen this in another clue)
'organisation with a return address' is the wordplay.
'organisation' becomes 'UN' (UN is an example).
'with' says to put letters next to each other.
'a return address' becomes 'arm' (I can't explain this - if you can you should give a lot more credence to this answer).
'un'+'arm'='UNARM'
Can you help me to learn more?
(Other definitions for unarm that I've seen before include "Remove weaponry from" , "Deprive of a weapon" , "Remove weapons from" , "Deactivate" , "Remove firing pieces" .)