That's right, the old-fashioned school head (3)

I believe the answer is:

yes

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'that's right' is the definition.
(right can mean yes or okay)

'old-fashioned school head' is the wordplay.
'old fashioned' becomes 'ye' (I can't justify this - if you can you should believe this answer much more).
'head' says to take the initial letters.
The first letter of 'school' is 's'.
'ye'+'s'='YES'

'the' acts as a link.

Can you help me to learn more?

(Other definitions for yes that I've seen before include "Certainly!" , "Agreed" , "Indeed" , "One response to "will you marry me?"" , "That's 27 [RIGHT]" .)

I've seen this clue in the Evening Standard.

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