Index tracker sheltering surplus (5)
I believe the answer is:
extra
'surplus' is the definition.
('extra' can be a synonym of 'surplus')
'index tracker sheltering' is the wordplay.
'sheltering' indicates the answer is hidden within the clue.
'EXTRA' is hidden in the letters of 'index tracker'.
(Other definitions for extra that I've seen before include "Possibly overthrow" , "particularly" , "fortuitous run" , "Actor in crowd scene, no lines" , "eg Wide; additional" .)