Possible Clues:

• Obstinate: hide when given new form mistress (1,3,2,3,4)
• Source of illicit fun — some here? (1,3,2,3,4)
• Relationship that might make you somewhat on edge?
• Active antibodies, the secret of a cheat? (1,3,2,3,4)
• Extramarital partner
• The antibodies destroyed illicit lover (1,3,2,3,4)
• Mistress, as sauce on a plate? (1,3,2,3,4)
• Mistress?
• One getting into bed with haste, all excited? (1,3,2,3,4)