Sound around shape initially absent in hypothetical tunnel (8)
I believe the answer is:
wormhole
'hypothetical tunnel' is the definition.
'sound around shape initially absent' is the wordplay.
'sound' becomes 'whole' ('whole' can be a synonym of 'sound').
'around' indicates putting letters inside.
'shape' becomes 'form' ('form' can be a synonym of 'shape').
'initially absent' means to remove the first letter.
'form' with its initial letter taken off is 'orm'.
'whole' placed around 'orm' is 'WORMHOLE'.
'in' acts as a link.
(Other definitions for wormhole that I've seen before include "Space made by burrowing creatures in fruit, wood, books etc" , "Hypothetical tunnel in space-time" , "Hypothetical link across space-time" , "Cavity make in wood by burrowing larvae" , "In theory, shortcut in space" .)