Line of cabs, first one leaving (4)
I believe the answer is:
axis
'line' is the definition.
(axis is a kind of line)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for axis that I've seen before include "Imaginary line about which a body rotates and a Dublin theatre" , "Imaginary central line" , "Central point of a rotation" , "Group of countries in special alliance" , "wartime alliance" .)