Cause one to be a time-server (8)

I believe the answer is:

imprison

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'cause' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how one could define the other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for imprison that I've seen before include "Put behind bars" , "Incarcerate" , "Put in gaol" , "Place in custody" , "Put away" .)

I've seen this clue in The Irish Independent.

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