Ultra efficient, but could be bettered (9)

I believe the answer is:

superable

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'could be bettered' is the definition.
I can't tell whether this definition defines the answer.

'ultra efficient' is the wordplay.
'ultra' becomes 'super' (I am not sure about this - if you are sure you should believe this answer much more).
'efficient' becomes 'able' (I've seen this before).
'super'+'able'='SUPERABLE'

'but' acts as a link.

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