Circular line that goes through the centre (8)

I believe the answer is:

diameter

I'm a little stuck... Click here to teach me more about this clue!

'circular line' is the definition.
'diameter' can be an answer for 'line' (diameter is a kind of line). I am not sure about the 'circular' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for diameter that I've seen before include "Width across the centre of a circle" , "Longest line in a circle" , "width of bar" , "Thickness, width (of a circle)" , "Remade it to width of a circle" .)

I've seen this clue in The Globe and Mail.

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