Earliest of books doesn't have enough on each significant body of water (5,3)
I believe the answer is:
black sea
'water' is the definition.
(Black Sea is an example)
'earliest of books doesn't have enough on each significant body' is the wordplay.
'earliest of' indicates taking the first letters.
'doesn't have' becomes 'lacks' (I've seen this before).
'enough on each significant body' becomes 'ea' (I can't explain this - if you can you should give a lot more credence to this answer).
The first letter of 'books' is 'b'.
'b'+'lacks'+'ea'='BLACK SEA'
'of' is the link.
Can you help me to learn more?
(Other definitions for black sea that I've seen before include "Eternally" , "Quantity of water" , "Body of water bounding Russia etc. - sounds murky" , "large amount of water" , "Tideless expanse of water between Russia, Ukraine etc - sounds murky" .)