Clue: Crafty greeting applies Henry of York sources (4)
Answer: AHOY
Explanation: Definition: Crafty greeting
First letters ('sources') of '**A**pplied **H**enry **O**f **Y**ork'. AHOY was used as a clandestine greeting by the Czechoslovak resistance in the Second World War, and has passed into general use in Czech, as both a greeting and a farewell.