Opening in the skin (4)
I believe the answer is:
pore
'opening in the skin' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pore that I've seen before include "Skin opening; look closely (over)" , "Closely study" , "Rope around the skin-opening" , "Small skin orifice; study" , "Sweat gland" .)