Most tedious restoration of The Elgin Marbles finally ahead of time (10)

I believe the answer is:

lengthiest

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'most tedious' is the definition.
Both the definition and answer are superlative adjectives. Perhaps you can see an association between them that I don't see?

'restoration of the elgin marbles finally ahead of time' is the wordplay.
'restoration of' indicates an anagram (the order of letters restored or reset).
'finally' says to take the final letters.
'ahead of' means one lot of letters go next to another (some letters go in front of others).
'time' becomes 't' (abbreviation).
The last letter of 'marbles' is 's'.
'the'+'elgin'='theelgin'
'theelgin' is an anagram of 'lengthie'.
'lengthie'+'s'+'t'='LENGTHIEST'

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I've seen this clue in The Independent.

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