I follow it through first bit, but not the last section (9)
I believe the answer is:
partition
'section' is the definition.
(thesaurus)
'i follow it through first bit but not the last' is the wordplay.
'follow' says to put letters next to each other.
'through' is an insertion indicator (inserted letters go through the word).
'first bit' becomes 'part one' (the first part).
'but not the last' means to remove the last letter.
'i' put after 'it' is 'iti'.
'partone' with its last letter removed is 'parton'.
'iti' going inside 'parton' is 'PARTITION'.
(Other definitions for partition that I've seen before include "Screen" , "wall" , "Barrier" , "segregation" , "Separation (of a country) into separate nations" .)