Open letters after a time (6)

I believe the answer is:

dilate

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'open' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for dilate that I've seen before include "Get bigger" , "Make (something) wider or more open" , "Enlarge or widen, like pupils perhaps" , "Widen in detail as pupils may" , "Make wider or larger" .)

I've seen this clue in the Evening Standard.

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