Per diems fiddled - I invested in waterproof layer? (9)

I believe the answer is:

epidermis

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'waterproof layer?' is the definition.
'epidermis' can be an answer for 'layer?' (epidermis is a kind of layer). I am unsure of the 'waterproof' bit.

'per diems fiddled i invested' is the wordplay.
'fiddled' indicates anagramming the letters.
'invested' is an insertion indicator.
'per'+'diems'='perdiems'
'perdiems' anagrammed gives 'epiderms'.
'epiderms' enclosing 'i' is 'EPIDERMIS'.

'in' is the link.

Can you help me to learn more?

(Other definitions for epidermis that I've seen before include "Outer layer of skin" , "Outermost layer" , "Protective covering" , "Surface of the skin" .)

I've seen this clue in The Independent.

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