Side motorists adopt left of imaginary line (5)
I believe the answer is:
axial
'of imaginary line' is the definition.
(relating to an axis)
'side motorists adopt left' is the wordplay.
'side' becomes 'xi'.
'motorists' becomes 'AA' (Automobile Association).
'adopt' means one lot of letters goes inside another (inserted letters are taken in or adopted).
'left' becomes 'L' (common abbreviation).
'xi' inserted into 'aa' is 'axia'.
'axia'+'l'='AXIAL'
(Other definitions for axial that I've seen before include "Central Line?" , "a couple of Poles?" , "Forming an axis" , "On a pivotal point" , "Like a 22 Across" .)