Fourth book nurse composed about doctor (7)
I believe the answer is:
numbers
'fourth book' is the definition.
(fourth book of the Bible)
'nurse composed about doctor' is the wordplay.
'composed' indicates an anagram.
'about' is an insertion indicator.
'doctor' becomes 'mb' (Bachelor of Medicine).
'nurse' with letters rearranged gives 'nuers'.
'nuers' going around 'mb' is 'NUMBERS'.
(Other definitions for numbers that I've seen before include "anaesthetists?" , "See 3" , "Amounts; counts" , "Fourth book of the Bible" , "Biblical book - anaesthetics" .)