Lambasting for what sounds like a second-class grade (8)
I believe the answer is:
berating
'lambasting' is the definition.
('berate' can be a synonym of 'lambast')
'what sounds like a second-class grade' is the wordplay.
'what sounds like' shows a homophone (sound like).
'a second class' becomes 'b'.
'grade' becomes 'rating' (I've seen this before).
'b' sounds like 'be'.
'be'+'rating'='BERATING'
'for' acts as a link.
(Other definitions for berating that I've seen before include "Reprimanding" , "Taking to task" , "Rebuking severely" , "Rebuking loudly" , "carpeting" .)