Making sure, or one will be wrong (9)
I believe the answer is:
erroneous
'wrong' is the definition.
(similar in meaning)
'making sure or one' is the wordplay.
'making' indicates anagramming the letters.
'sure'+'or'+'one'='sureorone'
'sureorone' with letters rearranged gives 'ERRONEOUS'.
'will be' acts as a link.
(Other definitions for erroneous that I've seen before include "not right" , "Mistaken, in error" , "Mistaken - inaccurate" , "Incorrect, wrong" , "with mistakes" .)