A spot of index-tracking makes more (5)

I believe the answer is:

extra

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

'more' is the definition.
(I've seen this before)

'a spot of index-tracking' is the wordplay.
'EXTRA' is hidden within the letters.
But, I'm unsure how this is indicated.

'makes' is the link.

Can you help me to learn more?

(Other definitions for extra that I've seen before include "additional supplies" , "An additional thing" , "Not a standard item" , "eg Wide; additional" , "Minor actor in crowd scene" .)

I've seen this clue in The Times.

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