Study the writer's tough stuff (5)
I believe the answer is:
denim
'tough stuff' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'study the writer's' is the wordplay.
'study' becomes 'den' (term for a study or office).
'the writer's' becomes 'im' ('I am' referring to the writer of the clue).
'den'+'im'='DENIM'
Can you help me to learn more?
(Other definitions for denim that I've seen before include "Coloured cotton twill" , "Twill cloth" , "Tough cotton fabric" , "Twilled cotton fabric" , "Hard-wearing cloth" .)