A surprised expression (7)

I believe the answer is:

strewth

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'expression' is the definition.
(I have seen 'Expression of surprise ' mean 'strewth' so perhaps 'expression' could also mean 'strewth')

I don't understand how the rest of the clue works.

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(Other definitions for strewth that I've seen before include "Exclamation of surprise" , "Mild expletive" , "Aussie exclamation" , "Antipodean exclamation" , "Said to be amazed" .)

I've seen this clue in the Evening Standard.

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