Initial data turning up in one set of books (5)
I believe the answer is:
input
'initial data' is the definition.
'turning up in one set of books' is the wordplay.
'turning' shows that the letters should be reversed in order.
'in' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'set of books' becomes 'nt' (New Testament).
'up' in reverse letter order is 'pu'.
'i'+'nt'='int'
'pu' going into 'int' is 'INPUT'.
(Other definitions for input that I've seen before include "Feed data into a computer" , "Contribution of work or information" , "Person's contribution (to a project)" , "Provision of data" , "Contribution; enter (data)" .)