Messing about on or close to barge sure is wrong (9)
I believe the answer is:
erroneous
'is wrong' is the definition.
(similar in meaning)
'messing about on or close to barge sure' is the wordplay.
'messing about' indicates an anagram.
'close to' says to take the final letters (the close of something is the end).
The final letter of 'barge' is 'e'.
'on'+'or'+'e'+'sure'='onoresure'
'onoresure' with letters rearranged gives 'ERRONEOUS'.
(Other definitions for erroneous that I've seen before include "Incorrect, wrong" , "not right" , "Mistaken - inaccurate" , "with mistakes" , "Mistaken, in error" .)