Pupils do it in the dark (6)

I believe the answer is:

dilate

'pupils do it in the dark' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for dilate that I've seen before include "Become wider or more open" , "Make or become wider" , "Expand in detail" , "Speak or write at length - widen" , "explain more fully" .)

I've seen this clue in the Evening Standard.

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