Flyer's fifth design? (5)
I believe the answer is:
plane
'flyer's' is the definition.
(plane is a kind of aircraft)
'fifth design?' is the wordplay.
I cannot really see how this works, but
'design?' could be 'plan' ('plan' can be a synonym of 'design') and 'plan' is located in the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for plane that I've seen before include "barked?" , "Woodworker's tool" , "Woodworking tool" , "Form of transport" , "Tool for smoothing wood" .)