Pub extended primarily behind region of vestibule (5,3)
I believe the answer is:
inner ear
'vestibule' is the definition.
(thesaurus)
'pub extended primarily behind region' is the wordplay.
'pub' becomes 'inn' (an inn is a type of pub).
'primarily' suggests taking the first letters.
'behind' means one lot of letters go next to another.
'region' becomes 'rear' (rear is a kind of region).
The first letter of 'extended' is 'e'.
'inn'+'e'+'rear'='INNER EAR'
'of' is the link.
(Other definitions for inner ear that I've seen before include "Auditor's part" , "vestibule and what it leads to" , "profound listener" , "Part of hearing organ" , "Where the semicircular canals are to be found" .)