Pacific region's one. A vast expanse before (7)
I believe the answer is:
oceania
'pacific region's' is the definition.
(region in the Pacific Ocean)
'one a vast expanse before' is the wordplay.
'one' becomes 'i' (Roman numeral).
'vast expanse' becomes 'ocean' (I've seen this in another clue).
'before' means one lot of letters go next to another.
'i'+'a'='ia'
'ia' after 'ocean' is 'OCEANIA'.
(Other definitions for oceania that I've seen before include "Orwellian superstate" , "group of islands" , "Islands of Pacific and adjacent seas" , "part of the world" , "Islands of the central and South Pacific" .)