Add to walled-in area (8)

I believe the answer is:

compound

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'add to' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for compound that I've seen before include "Combination of elements - make worse - fenced-in area" , "Fenced area; chemical substance" , "Blended substance - enclosed area" , "A mix of ingredients" , "Composed of two or more separate elements" .)

I've seen this clue in The Telegraph.

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