23 down
Lowest
point
of
brain
drain
with
regular
emigration
climbing
(5)
Lowest point of brain drain with regular emigration climbing (5)
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The definition is at the beginning of the clue.
How the clue works
See Lowest point sitting there as the definition — that's your NADIR, the exact opposite of the zenith. Now for the wordplay: pick out the regular letters of "brain drain" (every other letter) and you get R-I-D-A-N, then "climbing" tells you to flip that around, giving you NADIR.