Beatles favourite not long ago (9)

I believe the answer is:

yesterday

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'ago' is the definition.
Both the answer and definition are adverbs. Perhaps there's an association between them I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for yesterday that I've seen before include "24-hours ago" , "some time in the past" , "of most recent history" , "Recent time" , "Day before today" .)

I've seen this clue in The Telegraph.

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