According to Spooner, little Mildred acquired a diver (9)

I believe the answer is:

guillemot

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

I can't explain the rest of the clue.

(Other definitions for guillemot that I've seen before include "Seabird" , "cliff-ledge nester" , "An auk" , "Diver" , "Diving bird" .)

I've seen this clue in The Times.

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