European queen turning round in great circle (7)
I believe the answer is:
equator
'great circle' is the definition.
(equator is a kind of great circle)
'european queen turning round' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'queen' becomes 'qu' (abbreviation for queen).
'turning' becomes 'rota' (I can't explain this - if you can you should believe this answer much more).
'round' says the letters should be written backwards.
'rota' back-to-front is 'ator'.
'e'+'qu'+'ator'='EQUATOR'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for equator that I've seen before include "Imaginary line around the centre of the earth" , "Circle round the globe" , "Imaginary line around middle of the earth" , "Line separating northern and southern hemispheres" , "Line of zero latitude" .)