Expression of surprise about river smelling strongly (5)

I believe the answer is:

musky

'smelling strongly' is the definition.

'expression of surprise about river' is the wordplay.
'expression of surprise' becomes 'my' (I've seen this in other clues).
'about' indicates putting letters inside.
'river' becomes 'usk' (Welsh river).
'my' going around 'usk' is 'MUSKY'.

(Another definition for musky that I've seen is " Having strong smell".)

I've seen this clue in The Times.

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