Twice one's substituted for United? That's realistic (6)

I believe the answer is:

doable

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'realistic' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?

I don't understand the remainder of the clue.

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(Other definitions for doable that I've seen before include "possible" , "easy to achieve" , "Able to be carried out" , "Achievable, performable" .)

I've seen this clue in The Independent.

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