Crossword Genius

Calm son had extra tuition externally (10)

Ross

I believe the answer is:

restrained

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

I can't explain the rest of the clue.

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(Other definitions for restrained that I've seen before include "Checked" , "muted" , "Reserved, dispassionate" , "Being limited" , "Calm" .)

I've seen this clue in The Times.
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