Book about anger's boring (8)
I believe the answer is:
tiresome
'boring' is the definition.
('tiresome' can be a synonym of 'boring')
'book about anger's' is the wordplay.
'book' becomes 'tome' (tome is a kind of book).
'about' means one lot of letters goes inside another.
'anger's' becomes 'ires' ('ire' can be a synonym of 'anger').
'tome' placed around 'ires' is 'TIRESOME'.
(Other definitions for tiresome that I've seen before include "Annoying, being a nuisance" , "Annoying, makes you fed up" , "Exasperating" , "Fatiguing; boring" , "Meteors I find annoying" .)