Hi, rrrobbo, I’ll echo Meursault’s compliments for solving these puzzles without reference to Chambers!
Did you parse 33a in the end? The definition is “I’m industrious”, and the wordplay gives US for “me” surrounded (“having . . . cover”) by BYE (a type of “extra” in cricket). Two further letters (B and E) are in the opening of the thematic work and hence are omitted from the wordplay. I’m surprised that this expression isn’t in Chambers.
As regards the title, I thought at first that “it” referred to the rather dated, but popular with crossword setters, abbreviation SA for “sex appeal”, but I see that Chambers actually gives the first two words of the thematic phrase as a definition of “it”. So we could regard the thematic work as a note penned after “it” started. According to a biographer of the “narrator of the work” (referenced on Wikipedia), the thematic work was written after the author’s relationship with a long-standing friend became sexual, so the work could be regarded as a “post-it note” in that sense as well.