Port plug (4)
I believe the answer is:
cork
I believe this is a double definition.
'port' is the first definition.
(I've seen this before)
'plug' is the second definition.
(corking is a kind of plugging)
(Other definitions for cork that I've seen before include "One stops" , "Wine bottle plug" , "Irish city or county" , "Buoyant substance from a tree" , "Common bottle stopper" .)