Figures out a piece of linen is strong material (5)

I believe the answer is:

denim

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

'material' is the definition.
(type of fabric)

I can't explain the remainder of the clue.

(Other definitions for denim that I've seen before include "Tough cotton fabric" , "Cloth for jeans, etc" , "Material for casual trousers or jackets" , "Cotton twill fabric" , "Thick cotton cloth" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app