Form of address (6)
'form of address' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for title that I've seen before include "Name or designation" , "Name of, eg, book" , "Book's name" , "headline?" , "Identifying name" .)
(Other definitions for mister that I've seen before include "Bloke" , "Mr" , "Set rim (anag.)" , "that's what you'd call him" , "Male form of address" .)