Line round the earth (7)
I believe the answer is:
equator
'line round the earth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for equator that I've seen before include "Imaginary circle round the earth" , "Line separating northern and southern hemispheres" , "Line of 0 degrees latitude" , "Important line" , "Imaginary ring around the earth's centre" .)