Ibis are flying in vast region (7)
I believe the answer is:
siberia
'vast region' is the definition.
(I've seen this before)
'ibis are flying' is the wordplay.
'flying' indicates an anagram (letters fly around into new positions).
'ibis'+'are'='ibisare'
'ibisare' with letters rearranged gives 'SIBERIA'.
'in' acts as a link.
(Other definitions for siberia that I've seen before include "Area of eastern Russia" , "Vast Asian region of Russia" , "'Region of Russia, famous for its long, cold winters (7)'" , "Region to which Russian and then Soviet governments sent those considered disloyal" , "Arctic region" .)