Most showy marble dehydrates on time (9)

I believe the answer is:

tawdriest

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'most showy' is the definition.
Both the answer and definition are superlative adjectives. Maybe they are linked in a way I don't understand?

'marble dehydrates on time' is the wordplay.
'marble' becomes 'taw' (taw is a kind of marble).
'dehydrates' becomes 'dries' (I've seen this in another clue).
'on' is a charade indicator (letters next to each other).
'time' becomes 't' (abbreviation).
'taw'+'dries'+'t'='TAWDRIEST'

Can you help me to learn more?

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

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