Long extended piece unknown (7)

I believe the answer is:

lengthy

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This might be a double definition.

'long' is the first definition.
(synonyms)

'extended piece unknown' is the second definition.
'lengthy' can be an answer for 'extended' (thesaurus). I am not certain of the 'piece unknown' bit.

Can you help me to learn more?

(Other definitions for lengthy that I've seen before include "Of great duration" , "Taking up a lot of time" , "Prolix - tedious" , "rambling" , "Prolonged" .)

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

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