Membrane found defective in a primate (3,5)

I believe the answer is:

pia mater

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

'membrane' is the definition.
(pia mater is a kind of membrane)

'defective in a primate' is the wordplay.
'a primate' can be anagrammed to 'PIA MATER'.
But, I'm unsure how the anagram is indicated.

'found' acts as a link.

Can you help me to learn more?

(Another definition for pia mater that I've seen is " Membrane f".)

I've seen this clue in The Telegraph.

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