Reasons for putting some of us in boxes (6)
I believe the answer is:
causes
'reasons for' is the definition.
'causes' can be an answer for 'reasons' (the reasons for something). I'm not certain of the 'for' bit.
'some of us in boxes' is the wordplay.
'some of' says to take the initial letters (I've seen 'some' mean this).
'in' means one lot of letters goes inside another.
'boxes' becomes 'cases' (I can't justify this - if you can you should give a lot more credence to this answer).
The first letter of 'us' is 'u'.
'u' going into 'cases' is 'CAUSES'.
'putting' acts as a link.
Can you help me to learn more?
(Other definitions for causes that I've seen before include "See 5" , "They bring about effects" , "Reasons why things happen" , "Origins or crusaders" , "Gives rise to -- crusades" .)