Crossword Genius

Aid sure improved what's left (7)

Ross

I believe the answer is:

residua

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'left' is the definition.
I know nothing about this answer so I can't tell whether this works.

'aid sure improved' is the wordplay.
'improved' indicates anagramming the letters.
'aid'+'sure'='aidsure'
'aidsure' is an anagram of 'RESIDUA'.

'what's' is the link.

Can you help me to learn more?

(Another definition for residua that I've seen is " We've been left behind".)

I've seen this clue in The Daily Express.
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