Way of working will fill a second book (4)
I believe the answer is:
amos
'book' is the definition.
(Old Testament book)
'way of working will fill a second' is the wordplay.
'way of working' becomes 'mo' (abbreviation for modus operandi).
'will fill' is an insertion indicator.
'second' becomes 's' (common abbreviation).
'a'+'s'='as'
'mo' going inside 'as' is 'AMOS'.
(Other definitions for amos that I've seen before include "Part of bible" , "Old Testament book" , "Biblical character" , "Hebrew prophet" , "Old Testament prophet" .)