A security measure plugged by Conservative flyer from Canada (6)
I believe the answer is:
alcock
'flyer from canada' is the definition.
(John Alcock flew from Canada to Ireland in 1919)
'a security measure plugged by conservative' is the wordplay.
'security measure' becomes 'lock'.
'plugged by' indicates putting letters inside (plug can mean to block up a gap).
'conservative' becomes 'C' (UK political abbreviation).
'lock' going around 'c' is 'lcock'.
'a'+'lcock'='ALCOCK'
(Other definitions for alcock that I've seen before include "Flyer" , "one first crossed pond" , "Brown's Atlantic flying partner" , "British aviator who flew Atlantic with Brown" , "Aviator with 7 [BROWN]" .)