One left swapping places to sustain complaint (7)

I believe the answer is:

ailment

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'complaint' is the definition.
(synonyms)

I don't understand the remainder of the clue.

(Other definitions for ailment that I've seen before include "problem" , "Complaint, affliction" , "Malady" , "Bodily disorder or disease" , "infirmity" .)

I've seen this clue in The Sunday Times and The Times.

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