So superior, yet of little value (3)

I believe the answer is:

sou

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'yet of little value' is the definition.
I can't judge whether this definition 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 "Former small French coin" , "Something of little worth" , "Little money once" , "Very small amount of money" , "Paltry sum" .)

I've seen this clue in the Evening Standard.

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