X-Men given time to replace leader (3)

I believe the answer is:

ten

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'x' is the definition.
(X is the Roman numeral for ten)

I cannot understand how the remainder of the clue works.

(Other definitions for ten that I've seen before include "No of sides of a decagon" , "Very young" , "Half a score" , "PM's address, Downing St" , "Five times two" .)

I've seen this clue in The Independent.

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