So superior, yet of little cash value (3)

I believe the answer is:

sou

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'yet of little cash value' is the definition.
I can't tell whether this defines the answer.

'so superior' is the wordplay.
'superior' becomes 'u' (I've seen this in other clues).
'so'+'u'='SOU'

Can you help me to learn more?

(Other definitions for sou that I've seen before include "Little money once" , "Something of little worth" , "almost worthless" , "Defunct French coin" , "16 French coin" .)

I've seen this clue in the Evening Standard.

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