Crossword Genius

Wow: the famous Sir Walter (5,5)

Ross

I believe the answer is:

great scott

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'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" .)

I've seen this clue in The Guardian.
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