Some attend lessons -- too many to be counted (7)
I believe the answer is:
endless
'too many to be counted' is the definition.
(an endless or countless amount)
'some attend lessons' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ENDLESS' is hidden within 'attend lessons'.
(Other definitions for endless that I've seen before include "Everlasting" , "'Eternal, without finish (7)'" , "Continual" , "no boundaries" , "Occurring so frequently so as to seem uninterrupted" .)