Rectangular (6)
I believe the answer is:
oblong
'rectangular' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oblong that I've seen before include "Shape of a book" , "Elongated rectangle" , "Plane figure that deviates from a square by elongation" , "Oval" , "Rectangular two-dimensional figure" .)