Sound of a dove? I'm surprised! (3)
I believe the answer is:
coo
This clue is a double definition.
'sound of a dove?' is the first definition.
(I've seen this before)
'i'm surprised' is the second definition.
(exclamation of surprise)
(Other definitions for coo that I've seen before include "Cry of doves - cry of surprise?" , "Murmur amorously" , "Gosh!" , "crumbs" , "Expression of surprise (by a dove?)" .)