Focused on, when one taught (7)

I believe the answer is:

trained

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'one taught' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for trained that I've seen before include "I daren't (anag)" , "Did exercises" , "Worked out" , "Prepared for a task or career" , "Coached" .)

I've seen this clue in the Evening Standard.

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