Title is backed by publisher finally (3)
I believe the answer is:
sir
'title' is the definition.
(respectful address for a man)
'is backed by publisher finally' is the wordplay.
'backed' says the letters should be written in reverse.
'by' means one lot of letters go next to another.
'finally' suggests the final letters.
The last letter of 'publisher' is 'r'.
'is' written backwards gives 'si'.
'si'+'r'='SIR'
(Other definitions for sir that I've seen before include "Poet and novelist" , "Way to address gentleman" , "Formal and deferential address to a man" , "Title given to 10 Across" , "Feted fellow" .)