Ultimate piece of reproof in a long lecture (5)
I believe the answer is:
final
'ultimate' is the definition.
(similar in meaning)
'piece of reproof in a long lecture' is the wordplay.
'piece of' says the answer is hidden in the clue.
'FINAL' is hidden in the letters of 'reproof in a long lecture'.
(Other definitions for final that I've seen before include "Last, ultimate" , "binding" , "Last match in a competition" , "Concluding" , "Last in a series of games or contests" .)