To slip up 1-0 against us is wrong (9)
I believe the answer is:
erroneous
'is wrong' is the definition.
('erroneous' can be similar in meaning to 'wrong')
'to slip up 1-0 against us' is the wordplay.
'to slip up' becomes 'err' (I've seen this before).
'1' becomes 'one' ('one' can be a synonym of '1').
'0' becomes 'o' (I've seen this in other clues).
'against' is a charade indicator (letters next to each other).
'err'+'one'+'o'+'us'='ERRONEOUS'
(Other definitions for erroneous that I've seen before include "with mistakes" , "not right" , "Mistaken, in error" , "Mistaken - inaccurate" , "Incorrect, wrong" .)