Girl is a wow (3)
I believe the answer is:
amy
'girl' is the definition.
'a wow' is the wordplay.
'wow' becomes 'my' (both can be expressions of surprise).
'a'+'my'='AMY'
'is' acts as a link.
(Other definitions for amy that I've seen before include "- Johnson, Eng. aviatrix" , "Record-making British aviator, Johnson" , "--, Johnson, solo flyer" , "-- Johnson, female flyer" , "Youngest of Little Women" .)