Who was the female singer who was killed in a car crash and almost dacapitated?

You are probably referring to Jayne Mansfield who was an actress and not in the usual sense, a singer. She did record a few records but is mainly thought of as a more-than-amply endowed Actress. Accident, which may or may not have sheared off her head ( I am glad you said almost, as there are confusing stories) may have been indirectly caused by a smoke screen being laid down by a pest-abatement truck, which was hit but sustained only minor damage. Commence screening operations!- the countersign is- Holy Cow! ( pun intended).