Study one medium to heavy fabric (5)

I believe the answer is:

denim

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

'heavy fabric' is the definition.
'denim' can be an answer for 'fabric' (denim is a kind of fabric). I am not sure about the 'heavy' bit.

'study one medium' is the wordplay.
'study' becomes 'den' (term for a study or office).
'one' becomes 'i' (Roman numeral).
'medium' becomes 'm' (as in UK clothing sizes).
'den'+'i'+'m'='DENIM'

'to' is the link.

Can you help me to learn more?

(Other definitions for denim that I've seen before include "Twilled cotton fabric" , "Cotton twill fabric" , "Hard-wearing material seen in jeans" , "Hard-wearing blue fabric" , "Thick cotton cloth" .)

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

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