Illegal mailing (5,6)
I believe the answer is:
chain letter
'illegal mailing' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for chain letter that I've seen before include "one sent mushrooming?" , "The clarinet (anag.)" , "Fast-spreading communication" , "Cascading correspondence" , "Fraudulent scheme" .)