Boy has to cite wrong part (7)
I believe the answer is:
section
'part' is the definition.
(synonyms)
'boy has to cite wrong' is the wordplay.
'boy' becomes 'son' (synonyms).
'has' is an insertion indicator.
'wrong' indicates anagramming the letters.
'cite' with letters rearranged gives 'ecti'.
'son' enclosing 'ecti' is 'SECTION'.
(Other definitions for section that I've seen before include "Division, portion" , "surgical procedure" , "Cut" , "Segment" , "unit" .)