Ibis are assembled in vast region (7)
I believe the answer is:
siberia
'vast region' is the definition.
(I've seen this before)
'ibis are assembled' is the wordplay.
'assembled' is an anagram indicator.
'ibis'+'are'='ibisare'
'ibisare' anagrammed gives 'SIBERIA'.
'in' acts as a link.
(Other definitions for siberia that I've seen before include "Russian far-east" , "Region in former USSR noted for harsh winters and notorious for prison camps" , "Arctic region" , "Former place of exile for a Russian" , "'Region of Russia, famous for its long, cold winters (7)'" .)