1 across
Teacher
of
no
practical
use
(8)
Teacher of no practical use (8)
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The whole clue is the definition (& lit or double definition).
How the clue works
Two meanings bolted together here. Teacher gives you academic in the sense of a university lecturer type, while of no practical use is that other sense of academic — theoretical, purely hypothetical, no real-world bearing. Either definition gets you to the same word on its own.