Book unknown penned by English artist (4)
I believe the answer is:
ezra
'book' is the definition.
(book of the Bible)
'unknown penned by english artist' is the wordplay.
'unknown' becomes 'z' (algebra).
'penned by' indicates putting letters inside (inserted letters are penned in).
'english' becomes 'e' (abbreviation).
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'e'+'ra'='era'
'z' inserted inside 'era' is 'EZRA'.
(Other definitions for ezra that I've seen before include "High priest" , "OT prophet" , "Old Testament prophet" , "Eponymous scribe of the Old Testament" , "Old Testament book and rabbi" .)