Spy is not old enough to comprehend obscure books (10,5)
I believe the answer is:
undercover agent
'spy' is the definition.
(synonyms)
'not old enough to comprehend obscure books' is the wordplay.
'not old enough' becomes 'underage' (I've seen this in another clue).
'to comprehend' means one lot of letters goes inside another (comprehend can mean to contain or include).
'obscure' becomes 'cover' (I can't explain this - if you can you should give a lot more credence to this answer).
'books' becomes 'NT' (New Testament).
'underage' enclosing 'cover' is 'undercoverage'.
'undercoverage'+'nt'='UNDERCOVER AGENT'
'is' acts as a link.
Can you help me to learn more?
(Another definition for undercover agent that I've seen is " spy".)