Son in the morning quietly opens junk mail (4)
I believe the answer is:
spam
'junk mail' is the definition.
(I've seen this before)
'son in the morning quietly opens' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'in the morning' becomes 'am' (short for ante meridiem - before noon).
'quietly' becomes 'p' (p for piano and quiet).
'opens' indicates putting letters inside (some letters open a gap in others).
's'+'am'='sam'
'sam' going around 'p' is 'SPAM'.
(Other definitions for spam that I've seen before include "Unwanted communication" , "email messages" , "Tinned meat - unwanted email" , "Tinned processed meat" , "unwanted correspondence" .)