Nurse is out to ensnare doctor in the book (7)
I believe the answer is:
numbers
'book' is the definition.
(I've seen this before)
'nurse is out to ensnare doctor' is the wordplay.
'is out' indicates an anagram (out can mean wrong or inaccurate).
'to ensnare' is an insertion indicator.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus**).
'nurse' is an anagram of 'nuers'.
'nuers' placed around 'mb' is 'NUMBERS'.
'in the' acts as a link.
(Other definitions for numbers that I've seen before include "Fourth book of the Bible" , "Integers" , "They subdue the pain" , "Figures, digits" , "OT Book 4" .)