Learner overwhelmed by expression of surprise and merriment (4)
I believe the answer is:
glee
'merriment' is the definition.
(glee is a kind of merriment)
'learner overwhelmed by expression of surprise' is the wordplay.
'learner' becomes 'l'.
'overwhelmed by' indicates putting letters inside (I've seen this in other clues).
'expression of surprise' becomes 'gee' (I've seen this before).
'l' going into 'gee' is 'GLEE'.
'and' is the link.
(Other definitions for glee that I've seen before include "Mirth - part-song" , "Type of club" , "Great joy" , "Merriment or gloating" , "Unrestrained joy" .)