Response to expulsion by university admitting not so much (5,3)
I believe the answer is:
bless you
'response to expulsion' is the definition.
('bless you' is said after someone sneezes)
'by university admitting not so much' is the wordplay.
'university' becomes 'ou'.
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'not so much' becomes 'less' (I've seen this in another clue).
'by'+'ou'='byou'
'byou' going around 'less' is 'BLESS YOU'.
(Other definitions for bless you that I've seen before include "'It's said to a sneezer (5,3)'" , "Words said to a sneezer" , "Thanks a lot!" , "Wish to a sneezer" , "Prayer" .)