Area 51 fact: army uncloaked nine aliens, originally left of the birds in a region (9)
I believe the answer is:
avifaunal
'of the birds in a region' is the definition.
(relating to avifauna, ie the birds of a region)
'area 51 fact army uncloaked nine aliens originally left' is the wordplay.
I cannot quite see how this works, but
'area' could be 'a' (maths abbreviation) and 'a' is found within the answer.
'left' could be 'al' (I've seen this in other clues) and 'al' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
(Another definition for avifaunal that I've seen is " of local birds".)