Serious gain in the east when awful leader is deposed (7)

I believe the answer is:

earnest

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'serious' is the definition.
(similar in meaning)

I don't understand how the rest of the clue works.

(Other definitions for earnest that I've seen before include "guarantee" , "Sincere; enthusiastic" , "Grave" , "Serious; pledge" , "Impassioned" .)

I've seen this clue in The Irish Times.

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