Smoother, fifth design? (5)
I believe the answer is:
plane
'smoother' is the definition.
(I've seen this before)
'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 present in the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for plane that I've seen before include "Flat surface; tree" , "Tree - figure - aircraft" , "Smoother" , "Aircraft - level surface - carpenter's tool" , "that soars into the sky" .)