Substitute provided by keepers at zoo (6)
I believe the answer is:
'substitute' is the definition.
(I know that ersatz is a type of substitute)
'provided by keepers at zoo' is the wordplay.
'provided by' indicates a hidden word (the answer is provided within the clue).
'ERSATZ' is hidden in the letters of 'keepers at zoo'.
(Other definitions for ersatz that I've seen before include "'Artificial, not genuine (6)'" , "Made in imitation - in Germany" , "being of inferior quality" , "Not real or genuine" , "Imitation or substitute, from German" .)