On the other hand, some poetry appears in the first century only (10)

I believe the answer is:

conversely

'on the other hand' is the definition.
(I've seen this before)

'some poetry appears in the first century only' is the wordplay.
'some poetry' becomes 'verse' (synonyms).
'appears in' means one lot of letters goes inside another.
'the first century' becomes 'c' (1st letter of 'century').
'c'+'only'='conly'
'verse' going within 'conly' is 'CONVERSELY'.

(Another definition for conversely that I've seen is " On the other hand".)

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

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