Substitute Queen rested by entrance to zoo (6)

I believe the answer is:

ersatz

'substitute' is the definition.
(I know that substitute can be written as ersatz)

'queen rested by entrance to zoo' is the wordplay.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'rested' becomes 'sat' (to sit is to rest).
'by' is a charade indicator (letters next to each other).
'entrance to' says to take the initial letters.
The first letter of 'zoo' is 'z'.
'er'+'sat'+'z'='ERSATZ'

(Other definitions for ersatz that I've seen before include "Inferior substitute" , "Substitute - fake" , "Artificial or inferior imitation" , "Mock" , "Spurious" .)

I've seen this clue in The Guardian.

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