Wow: the famous Sir Walter (5,5)
I believe the answer is:
great scott
'wow' is the definition.
Both the answer and definition are singular nouns.
Maybe they are linked in a way I don't understand?
'famous sir walter' is the wordplay.
'famous' becomes 'great' (I've seen this in another clue).
'sir walter' becomes 'scott' (I've seen this in another clue).
'great'+'scott'='GREAT SCOTT'
'the' is the link.
Can you help me to learn more?
(Other definitions for great scott that I've seen before include "Expression of surprise after Sir Walter?" , "Good heavens!" , "Outburst" , "Exclamation of surprise" .)