Unwanted e-mail (4)
'unwanted e-mail' is the definition.
This is all the clue.
(Other definitions for spam that I've seen before include "Unwanted e-mails" , "Send unsolicited mail" , "unwanted delivery" , "Food" , "Unwanted communication" .)
(Other definitions for junk that I've seen before include "scrapped?" , "Clutter - Chinese boat" , "Abandon" , "ship" , "Accumulated rubbish" .)