An opening in dermatology (4)

I believe the answer is:

pore

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'an opening' is the definition.
(pore is a kind of opening)

I don't understand the rest of the clue.

(Other definitions for pore that I've seen before include "Sweat gland" , "Rope around the skin-opening" , "Small skin orifice; study" , "Sweat-gland opening" , "Minute passage in skin" .)

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