Possibly a quote by royal involving Earth's great circle (7)
I believe the answer is:
equator
'great circle' is the definition.
(I know that equator is a type of great circle)
'possibly a quote by royal' is the wordplay.
'possibly' is an anagram indicator.
'by' says to put letters next to each other.
'royal' becomes 'r' (abbreviation).
'a'+'quote'='aquote'
'aquote' with letters rearranged gives 'equato'.
'equato'+'r'='EQUATOR'
'involving earth's' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for equator that I've seen before include "Quorate (anag.)" , "It's equidistant from North and South Poles" , "Imaginary line round the earth" , "Line of zero latitude" , "Imaginary line dividing the earth's hemispheres" .)