Person writing schedule substitutes second half of month (8)
I believe the answer is:
novelist
'person writing' is the definition.
(someone who writes novels)
'schedule substitutes second half of month' is the wordplay.
I cannot really understand how this works, but
'schedule' could be 'list' (I've seen this in other clues) and 'list' is located in the answer.
'of' could be 'o' and 'o' is found in the answer.
This explanation may well be incorrect...
(Other definitions for novelist that I've seen before include "Source of books" , "Disraeli, perhaps" , "Spark produced, perhaps" , "Eliot or Bronte" , "King maybe" .)