One slimming takes a medium size? (8)

I believe the answer is:

diameter

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'size?' is the definition.
(thesaurus)

'one slimming takes a medium' is the wordplay.
'one slimming' becomes 'dieter' (I can't explain this - if you can you should believe this answer much more).
'takes' indicates putting letters inside.
'medium' becomes 'm' (as in UK clothing sizes).
'a'+'m'='am'
'dieter' enclosing 'am' is 'DIAMETER'.

Can you help me to learn more?

(Other definitions for diameter that I've seen before include "Longest chord of a circle" , "Thickness, width (of a circle)" , "It's equal to twice the radius" , "Two radii" , "Paying back (of money)" .)

I've seen this clue in The Telegraph.

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