Expression of surprise about learner showing mirth (4)
I believe the answer is:
glee
'mirth' is the definition.
(I've seen this before)
'expression of surprise about learner' is the wordplay.
'expression of surprise' becomes 'gee' (I've seen this before).
'about' indicates putting letters inside.
'learner' becomes 'l'.
'gee' enclosing 'l' is 'GLEE'.
'showing' acts as a link.
(Other definitions for glee that I've seen before include "Impish enjoyment" , "Elation" , "Great merriment" , "Old English part-song" , "Unholy mirth or triumphant joy" .)