Crossword Genius

Grid erroneously contains 1up (5)

Ross

I believe the answer is:

rider

I'm a little stuck... Click here to teach me more about this clue!

'1up' is the definition.
I can't tell whether this defines the answer.

'grid erroneously contains' is the wordplay.
'contains' indicates a hidden word.
'RIDER' is hidden within 'grid erroneously'.

Can you help me to learn more?

(Other definitions for rider that I've seen before include "Condition - under which a horse races" , "supported by an Arab, perhaps" , "Additional bit" , "Condition added to something already agreed" , "Addition to verdict" .)

I've seen this clue in The Independent.
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