Slate rocks on river in South African city (4,6)

I believe the answer is:

east london

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'south african city' is the definition.
The definition suggests a singular noun which matches the answer.

'slate rocks on river' is the wordplay.
'rocks' indicates anagramming the letters.
'river' becomes 'Don River' (Yorkshire river).
'slate' anagrammed gives 'eastl'.
'eastl'+'on'+'don'='EAST LONDON'

'in' is the link.

Can you help me to learn more?

(Other definitions for east london that I've seen before include "Poplar found in this" , "South African port?" , "Cockney part of the capital" .)

I've seen this clue in The Guardian.

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