Alphabetical key used to find extant investors (5)

I believe the answer is:

index

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

'alphabetical key' is the definition.
'index' can be an answer for 'key' (I've seen this before). I'm not certain of the 'alphabetical' bit.

'to find extant investors' is the wordplay.
'INDEX' is hidden within the letters.
But, I am unsure how this is indicated.

'used' acts as a link.

Can you help me to learn more?

(Other definitions for index that I've seen before include "Register of contents" , "List in back of book" , "Catalogue" , "Sign of something" , "Pointing finger" .)

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

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