Look to conceal terribly bitter text (8)
I believe the answer is:
libretto
'text' is the definition.
(I know that libretto is a type of text)
'look to conceal terribly bitter' is the wordplay.
'look' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'to conceal' means one lot of letters goes inside another.
'terribly' is an anagram indicator.
'bitter' is an anagram of 'ibrett'.
'lo' enclosing 'ibrett' is 'LIBRETTO'.
(Other definitions for libretto that I've seen before include "Script for a musical drama" , "Opera's words" , "textbook" , "Book of words of an opera" , "text for musicians" .)