One losing weight hides a metre thickness (8)

I believe the answer is:

diameter

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'thickness' is the definition.
(I've seen this before)

'one losing weight hides a metre' is the wordplay.
'one losing weight hides' becomes 'dia' (I can't explain this - if you can you should believe this answer much more).
'a metre' becomes 'meter' (synonyms).
'dia'+'meter'='DIAMETER'

Can you help me to learn more?

(Other definitions for diameter that I've seen before include "spans circle" , "A demerit for line bisecting a circle" , "Remade it to width of a circle" , "width of bar" , "Paying back (of money)" .)

I've seen this clue in The Sun.

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