Earnest acknowledgements of debt for faultless service (7)

I believe the answer is:

serious

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

I don't understand the remainder of the clue.

(Other definitions for serious that I've seen before include "Stern" , "Earnest - important" , "Solemn, thoughtful" , "Important -- grave" , "Earnest; vital" .)

I've seen this clue in the Evening Standard.

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