Some attend lessons non-stop (7)
I believe the answer is:
endless
'non-stop' is the definition.
(I've seen this before)
'some attend lessons' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'ENDLESS' is hidden within 'attend lessons'.
(Other definitions for endless that I've seen before include "Unceasing" , "'Eternal, without finish (7)'" , "Tiresomely long" , "Constant" , "Going on and on, interminable" .)