Overdue book caused delight (7)
I believe the answer is:
belated
'overdue' is the definition.
(I've seen this in another clue)
'book caused delight' is the wordplay.
I cannot really see how this works, but
'book' could be 'b' (abbreviation for book) and 'b' is present in the answer.
'delight' could be 'elate' (I've seen this before) and 'elate' is found in the answer.
A single letter 'd' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for belated that I've seen before include "After due time" , "Coming too late" , "Out of date" , "Coming after time" , "Behind time" .)