Expression of surprise about line in song (4)
I believe the answer is:
glee
'song' is the definition.
(I've seen this before)
'expression of surprise about line' is the wordplay.
'expression of surprise' becomes 'gee' (I've seen this before).
'about' means one lot of letters goes inside another.
'line' becomes 'l'.
'gee' enclosing 'l' is 'GLEE'.
'in' is the link.
(Other definitions for glee that I've seen before include "Part-song for male voices - delight" , "Old English part-song" , "Mirth - part-song" , "Elation" , "TV show" .)