Lad altered one book that's improvised (2-3)
I believe the answer is:
ad-lib
'improvised' is the definition.
(I've seen this in another clue)
'lad altered one book' is the wordplay.
'altered' indicates an anagram.
'one' becomes 'i' (Roman numeral).
'book' becomes 'b' (abbreviation for book).
'lad' with letters rearranged gives 'adl'.
'adl'+'i'+'b'='AD-LIB'
'that's' acts as a link.
(Other definitions for ad-lib that I've seen before include "Extemporise" , "Speak impromptu" , "Speak without preparation" , "Depart from the text" , "spontaneous comment" .)