Affectionate expression boy uses freely about student
I believe the answer is:
bless you
'affectionate expression' is the definition.
'boy uses freely about student' is the wordplay.
'freely' indicates an anagram.
'about' means one lot of letters goes inside another.
'student' becomes 'l'.
'boy'+'uses'='boyuses'
'boyuses' anagrammed gives 'bessyou'.
'bessyou' going around 'l' is 'BLESS YOU'.
(Other definitions for bless you that I've seen before include "'It's said to a sneezer (5,3)'" , "Prayer" , "Traditional saying to person who has sneezed" , "Reaction to mild explosion" , "remark following ejection?" .)