I'm ready for anything, so tackle book about origin of crosswords (4,2,2,2)
I believe the answer is:
sock it to me
'i'm ready for anything' is the definition.
(phrase used to encourage someone to a fight)
'so tackle book about origin of crosswords' is the wordplay.
'tackle' becomes 'kit' (both can mean gear or equipment).
'book' becomes 'tome' (tome is a kind of book**).
'about' says to put letters next to each other.
'origin of' suggests taking the first letters.
The initial letter of 'crosswords' is 'c'.
'kit'+'tome'='kittome'
'kittome' put after 'c' is 'ckittome'.
'so'+'ckittome'='SOCK IT TO ME'
(Other definitions for sock it to me that I've seen before include "Make a forceful impression on yours truly!" , "Tell yours truly in a vigorous manner!" .)