Express ignorance about Italian's title (6)
I believe the answer is:
'italian's title' is the definition.
(Italian equivalent to Mr)
'express ignorance about' is the wordplay.
'about' indicates a hidden word (the clue goes around or about the answer).
'SIGNOR' is hidden within 'express ignorance'.
(Other definitions for signor that I've seen before include "Italian gentleman" , "Title of Italian man" , "Title of Italian-speaking man" , "Italian mister" , "Guy in Rome" .)