In corner sat Zachary, substitute (6)

I believe the answer is:

ersatz

'substitute' is the definition.
(I know that ersatz is a type of substitute)

'in corner sat zachary' is the wordplay.
'in' indicates a hidden word.
'ERSATZ' is hidden in the letters of 'corner sat zachary'.

(Other definitions for ersatz that I've seen before include "Inferior substitute" , "being of inferior quality" , "Spurious" , "Made in imitation - in Germany" , "Imitation, not the real thing" .)

I've seen this clue in The Telegraph.

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