To berate top student is unfeeling (5)
I believe the answer is:
scold
'to berate' is the definition.
(synonyms)
'top student is unfeeling' is the wordplay.
'top' indicates taking the first letters (in a down clue, the first letter is top).
'is unfeeling' becomes 'cold' (I've seen this before).
The first letter of 'student' is 's'.
's'+'cold'='SCOLD'
(Other definitions for scold that I've seen before include "Rebuke; nagging woman" , "Chide - fishwife" , "Castigate" , "quarrelsome woman of old?" , "Chide - nag" .)