Completed penning opening of radical flyer (5)
I believe the answer is:
drone
'flyer' is the definition.
(drone is a kind of aircraft)
'completed penning opening of radical' is the wordplay.
'completed' becomes 'done' (I've seen this before).
'penning' means one lot of letters goes inside another.
'opening of' says to take the initial letters.
The first letter of 'radical' is 'r'.
'done' going around 'r' is 'DRONE'.
(Other definitions for drone that I've seen before include "Talk boringly; a parasite" , "Male bee for the queen" , "Male honey bee - hum" , "Remote controlled aircraft" , "boring speaker" .)