One-dimensional (6)
I believe the answer is:
linear
'one-dimensional' is the definition.
(I know that one-dimensional can be written as linear)
This is all the clue.
(Other definitions for linear that I've seen before include "following straight path" , "Of a line or length" , "In one dimension, I learn (anag.)" , "Long and narrow, of one dimension" , "Measured lengthwise" .)