Craft's fifth design (5)
I believe the answer is:
plane
'craft's' is the definition.
(thesaurus)
'fifth design' is the wordplay.
I cannot really see how this works, but
'design' could be 'plan' (synonyms) and 'plan' is found within the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for plane that I've seen before include "Aircraft - level surface - carpenter's tool" , "Smooth" , "Woodworking tool" , "Woodworker's tool" , "Flat surface; tree" .)