First book about a ruler is falling apart (8)
I believe the answer is:
breaking
'is falling apart' is the definition.
(I've seen this before)
'first book about a ruler' is the wordplay.
'first' is a charade indicator (letters next to each other).
'book about' becomes 'brea' (I am not sure about this - if you are sure you should believe this answer much more).
'a ruler' becomes 'king' (king is a kind of ruler).
'brea' next to 'king' is 'BREAKING'.
Can you help me to learn more?
(Other definitions for breaking that I've seen before include "Fracturing" , "Interrupting" , "Going first at snooker" , "falling apart" .)