Secretary into S&M gets unwanted mail (4)
I believe the answer is:
spam
'unwanted mail' is the definition.
(junk email)
'secretary into sm' is the wordplay.
'secretary' becomes 'PA' (abbreviation for personal assistant).
'into' is an insertion indicator.
'pa' inserted into 'sm' is 'SPAM'.
'gets' acts as a link.
(Other definitions for spam that I've seen before include "unsolicited communication" , "canned meat" , "Uninvited email" , "Possible item from menu" , "Tinned meat - unwanted email" .)