Unassuming engineer aims to leave without a fanfare (6)

I believe the answer is:

unsung

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

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

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(Other definitions for unsung that I've seen before include "Not honoured" , "Remarked by no-one" , "Without 1D [RECOGNITION]" , "Not given due recognition" , "Not properly acclaimed" .)

I've seen this clue in the Sydney Morning Herald.

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