Smashing book about what Charles has become? (8)
I believe the answer is:
breaking
'smashing' is the definition.
(smashing is a way of breaking something)
'book about what charles has become?' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'about' becomes 'rea' (I've seen this in other clues).
'what charles has become?' becomes 'king' (King Charles III).
'b'+'rea'+'king'='BREAKING'
(Other definitions for breaking that I've seen before include "Fracturing" , "Going first at snooker" , "Interrupting" , "falling apart" .)