Fail to keep old inside — it's immoral (5)
I believe the answer is:
loose
'it's immoral' is the definition.
(loose can mean promiscuous)
'fail to keep old' is the wordplay.
'fail' becomes 'lose' (I've seen this in other clues).
'to keep' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lose' going around 'o' is 'LOOSE'.
'inside' is the link.
(Other definitions for loose that I've seen before include "Baggy" , "Not secured" , "easy with one's favours" , "Unsecured; dissolute" , "Relaxed; set free" .)