Line of longitude (8)
I believe the answer is:
meridian
'line of longitude' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for meridian that I've seen before include "One passing through Greenwich" , "Imaginary great circle on the earth passing through both poles" , "Imaginary line of longitude" , "Imaginary great circle through North and South poles" , "Highest point" .)