Father engaging in kinky sex? I don’t want that in my inbox (4)
I believe the answer is:
spam
'i don't want that in my inbox' is the definition.
(spam is unwanted email)
'father engaging in kinky sex?' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'engaging in' is an insertion indicator (I've seen this in other clues).
'kinky sex?' becomes 'SM' (S and M, sadism and masochism).
'pa' inserted into 'sm' is 'SPAM'.
(Other definitions for spam that I've seen before include "Spiced ham" , "email messages" , "Send unsolicited mail" , "Possible item from menu" , "Tinned meat - unwanted email" .)