A book in the post is missing (6)
I believe the answer is:
absent
'is missing' is the definition.
(similar in meaning)
'a book in the post' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'in the post' becomes 'sent' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'b'+'sent'='ABSENT'
Can you help me to learn more?
(Other definitions for absent that I've seen before include "Missing, lacking" , "Forgetful, with mind on other things" , "lack of attention" , "Not where one should be" , "Missing or dreamy" .)