Fathom excellent book (5)
I believe the answer is:
plumb
'fathom' is the definition.
(I've seen this before)
'excellent book' is the wordplay.
'excellent' becomes 'plum' (I can't justify this - if you can you should give a lot more credence to this answer).
'book' becomes 'b' (abbreviation for book).
'plum'+'b'='PLUMB'
Can you help me to learn more?
(Other definitions for plumb that I've seen before include "Utterly" , "Lead weight for measuring depth" , "Fathom - out-and-out" , "Going straight up and down" , "Reach or experience, say the depths" .)