Added restriction of index-tracker (5)

I believe the answer is:

extra

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'added restriction' is the definition.
'extra' can be an answer for 'added' (I've seen this in another clue). I am not sure about the 'restriction' bit.

'of index-tracker' is the wordplay.
'of' indicates a hidden word.
'EXTRA' is hidden in the letters of 'index tracker'.

Can you help me to learn more?

(Other definitions for extra that I've seen before include "Ancillary" , "Small part actor" , "eg Wide; additional" , "An increased amount" , "One playing small part" .)

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

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