Witness record found in complete set of books (8)
I believe the answer is:
deponent
'witness' is the definition.
(I know that deponent is a type of witness)
'record found in complete set of books' is the wordplay.
'record' becomes 'EP' (abbreviation for extended play record).
'found in' means one lot of letters goes inside another.
'complete' becomes 'done' (I've seen this in another clue).
'set of books' becomes 'NT' (New Testament).
'ep' put into 'done' is 'depone'.
'depone'+'nt'='DEPONENT'
(Other definitions for deponent that I've seen before include "Person giving evidence under oath" , "One making a statement under oath" .)