Reason radioactive substance is boxed (5)
I believe the answer is:
cause
'reason' is the definition.
(I've seen this before)
'substance is boxed' is the wordplay.
'substance' becomes 'Ca' (Ca is an example).
'is boxed' becomes 'use' (I can't justify this - if you can you should believe this answer much more).
'ca'+'use'='CAUSE'
'radioactive' is the link.
This may not be correct. It may belong to another bit of the clue.
(Other definitions for cause that I've seen before include "why some people fight" , "Producer of an effect" , "A movement or charity- usually good" , "Reason; belief" , "Produce (an effect)" .)