Amniotic membrane (4)
I believe the answer is:
caul
'amniotic membrane' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for caul that I've seen before include "Membrane covering an infant's head" , "It covers the head at birth" , "Membrane sometimes on head of new-born baby" , "Membrane found on child's head at birth, considered lucky" , "Membrane around fetus" .)