Question

an ace in the hole
 an ace in the hole  (US)   ( or chiefly Brit   an ace up your sleeve)
  ◇ If you have an ace in the hole or an ace up your sleeve, you have a powerful and often secret weapon, advantage, etc., that you can use if it is needed.
   The mayor's popularity among elderly voters gives him an ace in the hole for the coming election.
  
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 Main Entry: ace
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 an ace in the hole — see ace, 1
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 Main Entry: hole