I tape cork up in cavity full of gas (3,6)
I believe the answer is:
air pocket
'cavity full of gas' is the definition.
(pocket of gas)
'i tape cork up' is the wordplay.
'up' indicates an anagram (letters get chucked up).
'i'+'tape'+'cork'='itapecork'
'itapecork' is an anagram of 'AIR-POCKET'.
'in' is the link.
(Other definitions for air pocket that I've seen before include "local region of low pressure" , "Vacuum caused by an atmospheric down-draught" , "Local area of low pressure that causes a plane to lose height suddenly" .)