Means to exchange trousers quietly with son (6)

I believe the answer is:

spells

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But I can't explain it at all! Could you help me understand?

(Other definitions for spells that I've seen before include "Lists characters" , "period pieces" , "Formulae believed to have magical force" , "Magic formulae" , "Trances" .)

I've seen this clue in The Independent.

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