Sound of amazement (3)
I believe the answer is:
ooh
'sound of amazement' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ooh that I've seen before include "Expression of surprise" , "Exclamation of delight" , "sound excited" , "Exclamation of surprise or pain" , "My goodness!" .)