The first sort of test in old solver's book (7)
I believe the answer is:
timothy
'book' is the definition.
(I know that Timothy is a Epistle)
'the first sort of test in old solver's' is the wordplay.
'the first' becomes 'i' (I in Roman numerals eg Elizabeth I).
'sort of test' becomes 'MOT' (MOT test for a vehicle).
'in' is an insertion indicator.
'old solver's' becomes 'thy' (belonging to thee, the crossword solver).
'i'+'mot'='imot'
'imot' put within 'thy' is 'TIMOTHY'.
(Other definitions for timothy that I've seen before include "haymaker?" , "Companion to St.Paul; type of grass" , "Bible book" , "Grass; boy's name" , "A grass; an NT disciple" .)