Fellow altered middle section (7)
I believe the answer is:
chapter
'section' is the definition.
(I know that chapter is a type of section)
'fellow altered middle' is the wordplay.
'fellow' becomes 'chap' (both can mean a man).
'middle' says to take the centre.
The middle of 'altered' is 'ter'.
'chap'+'ter'='CHAPTER'
(Other definitions for chapter that I've seen before include "religious assembly" , "Sequence - section of book - church body" , "Part of book" , "series of events" , "Division of book, usually numbered" .)