Large bitter drunk by head of Indigo Books (8)
I believe the answer is:
libretti
'books' is the definition.
(libretto is a kind of book)
'large bitter drunk by head of indigo' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'drunk' indicates an anagram.
'by' says to put letters next to each other.
'head of' says to take the initial letters.
The first letter of 'indigo' is 'i'.
'bitter' is an anagram of 'ibrett'.
'l'+'ibrett'+'i'='LIBRETTI'
(Other definitions for libretti that I've seen before include "Books" , "score-books?" , "Da Ponte's texts" , "Opera texts" , "Words" .)