A second set of books required on Charlie's course (5)
I believe the answer is:
ascot
'course' is the definition.
(Ascot is a racecourse)
'a second set of books required on charlie's' is the wordplay.
'second' becomes 's' (common abbreviation).
'set of books' becomes 'OT' (abbreviation for the Old Testament).
'required on' says to put letters next to each other.
'charlie' becomes 'C' (phonetic alphabet).
'ot' put after 'c' is 'cot'.
'a'+'s'+'cot'='ASCOT'
(Other definitions for ascot that I've seen before include "Town with racecourse" , "Tie round the neck" , "English course" , "somewhere in Berks" , "sporting event" .)