Exclamation of amazement (3)
(Other definitions for wow that I've seen before include "Exclamation of surprise or praise - a palindrome" , "Palindromic exclamation of surprise or admiration" , "Expression of great admiration" , "Expression of surprised admiration" , "Great success" .)
'exclamation of amazement' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aah that I've seen before include "Expression of appreciation or amazement" , "regular partner of 3 [OOH]" , "Expression of surprise" .)