It's in the upper third of GB (5)
I believe the answer is:
perth
'gb' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'it's in the upper third of' is the wordplay.
'it's in' indicates the answer is hidden within the clue.
'PERTH' is hidden within 'upper third of'.
Can you help me to learn more?
(Other definitions for perth that I've seen before include "Western Australia's capital" , "Period (of effort)" , "Town at the head of the Tay estuary" , "Australian or Scottish city" , "Australian state capital" .)