People start to talk in lessons (7)
I believe the answer is:
mortals
'people' is the definition.
('mortal' can be a synonym of 'person')
'start to talk in lessons' is the wordplay.
'start to' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'lessons' becomes 'morals' ('moral' can be a synonym of 'lesson').
The initial letter of 'talk' is 't'.
't' inserted into 'morals' is 'MORTALS'.
(Other definitions for mortals that I've seen before include "People - lost arm (anag)" , "Lord, what fools these -- be! (AMND)" .)