Alert to working train attendant (2,5)

I believe the answer is:

on guard

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

'working train attendant' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'train attendant' becomes 'guard' (I am not sure about this - if you are sure you should believe this answer much more).
'on'+'guard'='ON GUARD'

'to' acts as a link.

Can you help me to learn more?

(Other definitions for on guard that I've seen before include "Alert, vigilant" , "Wary" , "Alert like sentry" , "Keeping watch" , "Watchful" .)

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

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