Amazing doctors' union lost enthusiasm (4)

I believe the answer is:

zing

I'm a little stuck... Click here to teach me more about this clue!

'enthusiasm' is the definition.
(I've seen this in another clue)

'amazing doctors' union lost' is the wordplay.
'ZING' is hidden within the letters.
However, I'm uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for zing that I've seen before include "Energy, enthusiasm" , "Energy - liveliness" , "Go" , "Dynamism" , "Vigour" .)

I've seen this clue in the Sydney Morning Herald.

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