Opened up late and completed the exterior (7)

I believe the answer is:

dilated

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'opened up' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for dilated that I've seen before include "like pupils in the dark" , "Made more open/wide" , "Widened; expanded" , "Enlarged" , "Add tile (anag)" .)

I've seen this clue in the Evening Standard.

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