Back in time Chinese feel pained (7)

I believe the answer is:

agonise

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

I can't explain the remainder of the clue.

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(Other definitions for agonise that I've seen before include "Labour" , "Fret, worry inordinately" , "Struggle mentally" , "Suffer great pain over something" , "Undergo great anguish" .)

I've seen this clue in The Guardian and The Sun.

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