Protective cover? There's audible demand for that (4)

I believe the answer is:

caul

I'm a little stuck... Click here to teach me more about this clue!

'protective cover?' is the definition.
(I know that caul is a type of cap)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for caul that I've seen before include "Membrane found on child's head at birth, considered lucky" , "Membrane around fetus" , "Membrane sometimes on child's head at birth" , "Membrane covering an infant's head" , "Membrane sometimes on head of new-born baby" .)

I've seen this clue in The Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.