In middle of lesson get unfriendly rebukes (6)
I believe the answer is:
scolds
'rebukes' is the definition.
('scold' can be a synonym of 'rebuke')
'in middle of lesson get unfriendly' is the wordplay.
'in middle of lesson' becomes 'ss' (I am not sure about this - if you are sure you should believe this answer much more).
'get' is an insertion indicator.
'unfriendly' becomes 'cold' ('cold' can be similar in meaning to 'unfriendly').
'ss' going around 'cold' is 'SCOLDS'.
Can you help me to learn more?
(Other definitions for scolds that I've seen before include "Censures, berates" , "Reprimands, tells off harshly" , "Chides, reprimands" , "Tells off, rebukes" , "Dresses down" .)