Central line (4)
I believe the answer is:
axis
'central line' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for axis that I've seen before include "World War two Italy-Germany alliance" , "Alliance of powers eg Germany, Italy and Japan in WWII" , "Turning point" , "Feasible" , "Fixed line in revolution" .)