A series of books about bovine disease is missing (6)
I believe the answer is:
absent
'is missing' is the definition.
('absent' can be similar in meaning to 'missing')
'a series of books' is the wordplay.
'a series of' becomes 'abse' (I can't explain this - if you can you should believe this answer much more).
'books' becomes 'nt' (New Testament).
'abse'+'nt'='ABSENT'
'about bovine disease' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for absent that I've seen before include "nowhere to be seen" , "Not at a specified place" , "Missing, lacking" , "Inattentive" , "lack of attention" .)