It's in the upper third of the UK (5)

I believe the answer is:

perth

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'uk' is the definition.
The definition and answer can be both locations as well as being singular nouns.
Maybe they are linked in a way 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'.

'the' is the link.

Can you help me to learn more?

(Other definitions for perth that I've seen before include "Scottish/Australian city" , "Period (of effort)" , "Home of fair maid" , "Australian state capital" , "City of Scotland" .)

I've seen this clue in the Evening Standard.

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