River of splashy froth? (5)

I believe the answer is:

forth

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'river' is the definition.
(Scottish river)

'splashy froth?' is the wordplay.
'froth' anagrams to 'FORTH'.
But, I am unsure how this is indicated.

'of' acts as a link.

Can you help me to learn more?

(Other definitions for forth that I've seen before include "River and estuary in eastern Scotland" , "running into view" , "Scottish river and firth" , "Onward direction" , "Onward; the Firth of - -" .)

I've seen this clue in the Evening Standard.

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