Disturbed, seeing end of libellous book (7)
I believe the answer is:
genesis
'book' is the definition.
(first book of the Bible)
'disturbed seeing end of libellous' is the wordplay.
'disturbed' indicates anagramming the letters (the order of letters is disturbed).
'end of' indicates one should take the final letters.
The last letter of 'libellous' is 's'.
'seeing' anagrammed gives 'genesi'.
'genesi'+'s'='GENESIS'
(Other definitions for genesis that I've seen before include "First book of the Old Testament" , "Old Testament Book 1" , "Creation" , "Origin (of the Old Testament?)" , "Origin; Bible book" .)