Strategy switching first two pieces in Xs and Os? (6)

I believe the answer is:

tactic

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'strategy' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

(Other definitions for tactic that I've seen before include "A manoeuvre or planned action" , "Planned manoeuvre or skilful move" , "Action aimed at a specific end" , "Plan, manoeuvre" , "Device" .)

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

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