Marseilles lacking a rebuild becomes more pungent (8)

I believe the answer is:

smellier

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

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

Can you help me to learn more?

(Another definition for smellier that I've seen is " More pongy".)

I've seen this clue in The Irish Times.

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