Grid erroneously contains 1up (5)
I believe the answer is:
rider
'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" .)