Slippery individual sacking ninety (7)

I believe the answer is:

elusive

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

I don't understand how the rest of the clue works.

(Other definitions for elusive that I've seen before include "Difficult to grasp" , "Difficult to understand" , "Use veil to be difficult to catch" , "Difficult to catch, hard to pin down" , "Evil use (anag.)" .)

I've seen this clue in The Sun.

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