Let them know if you can but not if you start (6)

I believe the answer is:

notify

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'let them know if you can' is the definition.
I can't tell whether this definition defines the answer.

'not if you start' is the wordplay.
'you start' becomes 'y' (I've seen this in other clues).
'not'+'if'+'y'='NOTIFY'

'but' acts as a link.

Can you help me to learn more?

(Other definitions for notify that I've seen before include "Officially inform" , "Send word" , "'Advise, tell about (6)'" , "Formally inform" , "Advise, apprise" .)

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

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