Crossword Genius

Radius partly divided by number or back number (8)

Ross

I believe the answer is:

dinosaur

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'back number' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'radius partly divided by number' is the wordplay.
'radius partly' becomes 'disaur' (I can't justify this - if you can you should believe this answer much more).
'divided by' indicates putting letters inside.
'number' becomes 'no' (abbreviation).
'disaur' going around 'no' is 'DINOSAUR'.

'or' is the link.

Can you help me to learn more?

(Other definitions for dinosaur that I've seen before include "Extinct monster like Barney, children's TV pal" , "Enormous extinct reptile of ancient times" , "One as T. Rex, triceratops" , "Extinct reptile of enormous size" , "old-fashioned sort" .)

I've seen this clue in the Sydney Morning Herald.
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