Repeat lecturer unexpectedly shrunk somewhat (5)
I believe the answer is:
rerun
'repeat' is the definition.
(I've seen this before)
'lecturer unexpectedly shrunk somewhat' is the wordplay.
'somewhat' indicates a hidden word.
'RERUN' can be found hidden inside 'lecturer unexpectedly shrunk'.
(Other definitions for rerun that I've seen before include "Put on again" , "we've seen that before" , "Repeat of event" , "Film or programme broadcast again" , "Performed or broadcast again" .)