Used rail wrongly and left (8)

I believe the answer is:

residual

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'left' is the definition.
Although both the answer and definition are adjectives, I can't understand how they can define each other.

'used rail wrongly' is the wordplay.
'wrongly' is an anagram indicator.
'used'+'rail'='usedrail'
'usedrail' anagrammed gives 'RESIDUAL'.

'and' is the link.

Can you help me to learn more?

(Other definitions for residual that I've seen before include "Leftover - used rail (anag)" , "unused" , "Pertaining to what is left over" , "Remaining at the end" , "surplus" .)

I've seen this clue in The Sun.

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