### Clue: A security measure plugged by Conservative flyer from Canada (6)
### Answer: ALCOCK 
 ### Explanation: Definition: flyer from Canada
An envelope ('plugged by') of C ('Conservative') in 'a' plus LOCK ('security measure'), for John ALCOCK, who was British, but, with navigator Arthur Whitten Brown, flew the first non-stop transatlantic flight from Newfoundland to Ireland.