17 across
Former
student
extracting
iodine
from
metallic
oxide
(6)
Former student extracting iodine from metallic oxide (6)
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The definition is at the beginning of the clue.
How the clue works
The definition is Former student — specifically a female graduate. For the wordplay, take metallic oxide ALUMINA (that's aluminium oxide), then extracting iodine means removing the I (iodine's chemical symbol), and you're left with ALUMNA.