The most common and most likely answer to this crossword clue is the 7 letter word KINROSS
I've seen this crossword clue in The Guardian
K
I
N
R
O
S
S
More clues leading to the result KINROSS
Buddy explains!
The straight part
We know that "old borough south of Perth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old borough south of perth = kinross.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.