One Butlins resort could be integral (2-5)

I believe the answer is:

in-built

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'be integral' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for in-built that I've seen before include "Incorporated as part of structure" , "Constructed as integrated part" , "Existing as an essential constituent" , "Integral" .)

I've seen this clue in The Telegraph.

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