Arid region has receding external borders (5)

I believe the answer is:

sahel

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'arid region' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'has receding external borders' is the wordplay.
'receding' is a reversal indicator.
'borders' says to hollow out the word (remove centre letters) (only the edge or border letters remain).
'external' with its centre taken out is 'el'.
'has' written backwards gives 'sah'.
'sah'+'el'='SAHEL'

Can you help me to learn more?

(Other definitions for sahel that I've seen before include "African belt" , "heals wound" , "Semi-arid region of West Africa" , "Region to south of Sahara desert" .)

I've seen this clue in the Sydney Morning Herald.

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