Crossword Genius

Discussion item about radius line on a globe (6)

Ross

I believe the answer is:

tropic

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'discussion item' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'radius line on a globe' is the wordplay.
'radius' becomes 'r' (abbreviation in geometry).
'line' indicates putting letters inside.
'on' becomes 'to'.
'a globe' becomes 'pic' (I can't justify this - if you can you should believe this answer much more).
'r' put within 'to' is 'tro'.
'tro'+'pic'='TROPIC'

'about' is the link.

Can you help me to learn more?

(Other definitions for tropic that I've seen before include "Cancer is one, Capricorn another" , "Line bordering hot region" , "One of two parallels, Cancer or Capricorn" , "Limiting line of torrid zone" , "Parallel of latitude about 25o N or S" .)

I've seen this clue in The Times.
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