A line of taxis (4)
I believe the answer is:
axis
'a line' is the definition.
(axis is a kind of line)
'taxis?' is the wordplay.
'AXIS' is hidden within the letters.
But, I'm uncertain how this is indicated.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for axis that I've seen before include "Centre about which something turns" , "1936 alliance between Germany and Italy" , "Line from N. to S. Pole" , "Imaginary central line" , "World War two Italy-Germany alliance" .)