2 down
It’s
coming
up
in
newest
development
of
the
decade
(8)
It’s coming up in newest development of the decade (8)
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The definition is at the end of the clue.
How the clue works
Look at of the decade — that's your definition, since the twenties is indeed a decade. For the wordplay, take IT "coming up" (reversed to give TI) and stick it together with NEWEST, then "development" signals an anagram of all eight letters, giving you TWENTIES.