25 across
Teaching
the
students
at
last,
mathematician
is
using
body
language
(9)
Teaching the students at last, mathematician is using body language (9)
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The definition is at the end of the clue.
How the clue works
Using body language is your definition here. Take the last letters of Teaching, the, students (that's G, E, S — "at last" doing the heavy lifting) and stick them in front of mathematician TURING (Alan, naturally) to get the whole thing.