Crossword Genius

What might make you stir alive if slack? (9)

Ross

I believe the answer is:

vitaliser

'slack?' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'what might make you stir alive' is the wordplay.
'stir alive' can be anagrammed to 'VITALISER'.
But, I'm unsure how this is indicated.

'if' acts as a link.

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