Geometry surface (5)
I believe the answer is:
plane
'geometry surface' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for plane that I've seen before include "This flier" , "Tree - figure - aircraft" , "Woodworker's tool" , "Aircraft - level surface - carpenter's tool" , "that soars into the sky" .)