What’s offered in school is lesson material (5)
I believe the answer is:
lisle
'what's offered' is the definition.
I can't judge whether this defines the answer.
'in school is lesson material' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LISLE' is hidden in the letters of 'school is lesson material'.
Can you help me to learn more?
(Other definitions for lisle that I've seen before include "Cotton stocking thread" , "Strong cotton material" , "Strong cotton fabric" , "Type of cotton yarn" , "Fine smooth cotton thread for stockings" .)