It's in the upper third of Western Australia (5)
I believe the answer is:
perth
'western australia' is the definition.
The definition and answer can be both locations as well as being singular nouns.
Maybe there's an association between them I don't understand?
'it's in the upper third of' is the wordplay.
'it's in' indicates a hidden word.
'PERTH' is hidden in the letters of 'upper third of'.
Can you help me to learn more?
(Other definitions for perth that I've seen before include "Home of fair maid" , "Australian state capital" , "Australian or Scottish city" , "Capital of Western Australia" , "Period (of effort)" .)