Youngster I start to lecture? (5)
I believe the answer is:
pupil
'youngster i start to lecture?' is the definition.
I can't judge whether this definition defines the answer.
'youngster i start to lecture?' is the wordplay.
'youngster' becomes 'pup' (I've seen this before).
'start to' indicates taking the first letters.
The initial letter of 'lecture' is 'l'.
'pup'+'i'+'l'='PUPIL'
(Other definitions for pupil that I've seen before include "The sort of student" , "Eye hole" , "one may see through!" , "Learner in the eye?" , "part of my anatomy" .)