Doctor absorbed in a second book (4)
I believe the answer is:
amos
'book' is the definition.
(Old Testament book)
'doctor absorbed in a second' is the wordplay.
'doctor' becomes 'mo' (medical officer**).
'absorbed in' indicates putting letters inside.
'a second' becomes 'as' (I've seen this in other clues).
'mo' going into 'as' is 'AMOS'.
(Other definitions for amos that I've seen before include "30th OT book" , "Biblical character" , "Biblical prophet and OT book" , "Prophet in the Bible" , "Old Testament prophet" .)