Tiny opening in the skin (4)
I believe the answer is:
pore
'tiny opening in the skin' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pore that I've seen before include "Minute passage in skin" , "Small opening in the skin" , "Minute opening in the skin" , "Rope around tiny hole" , "Rope around the skin-opening" .)