Moira is employed to keep Jack up (5)

I believe the answer is:

raise

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'jack up' is the definition.
(to jack something up is to raise or lift it)

I can't explain the rest of the clue.

(Other definitions for raise that I've seen before include "Lift up, bring up" , "Lift up or rear" , "Inflate" , "Collect - increase in pay" , "Hoist" .)

I've seen this clue in The Sun.

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