Ultimate piece of reproof in a long lecture (5)
I believe the answer is:
final
'ultimate' is the definition.
('final' can be similar in meaning to 'ultimate')
'of reproof in a long lecture' is the wordplay.
'of' indicates the answer is hidden within the clue.
'FINAL' can be found hidden inside 'reproof in a long lecture'.
'piece' acts as a link.
(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" .)