No part of the line is straight (5)
I believe the answer is:
curve
'no part of the line is straight' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for curve that I've seen before include "Line on a graph" , "way of learning" , "Line that's not straight" , "No part of this line is straight" , "that may be hyperbolic" .)