Cardinal possibly reading an Old Testament book (7)
I believe the answer is:
numbers
'old testament book' is the definition.
(I've seen this before)
'cardinal possibly reading an' is the wordplay.
'cardinal possibly' becomes 'number' (cardinal is a kind of number).
'reading an' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'number'+'s'='NUMBERS'
Can you help me to learn more?
(Other definitions for numbers that I've seen before include "Fourth book of the Bible" , "Old Testament book -- narcotics?" , "Integers" , "See 3" , "Fourth book of the Old Testament" .)