Section of diagram is surely wrong (5)
I believe the answer is:
amiss
'wrong' is the definition.
(synonyms)
'section of diagram is surely' is the wordplay.
'section of' indicates a hidden word.
'AMISS' is hidden in the letters of 'diagram is surely'.
(Other definitions for amiss that I've seen before include "Not functioning properly" , "Mistakenly" , "inappropriately" , "In an inappropriate way" , "error" .)