Society girl American knocked up in the sack (4)
I believe the answer is:
abed
'in the sack' is the definition.
'society girl american knocked up' is the wordplay.
'society girl' becomes 'deb' (I've seen this before).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'knocked up' says the letters should be written backwards.
'deb'+'a'='deba'
'deba' backwards is 'ABED'.
(Other definitions for abed that I've seen before include "Possibly sleeping" , "in retirement, possibly?" , "Yet to rise" , "Sleeping under covers" , "how we all hope to go?" .)