Difficult question about television's first advertisement (6)
I believe the answer is:
poster
'advertisement' is the definition.
(poster is a kind of advertisement)
'difficult question about television's first' is the wordplay.
'difficult question' becomes 'poser' (I've seen this in another clue).
'about' is an insertion indicator.
'television's first' becomes 't' (1st letter of 'television').
'poser' going around 't' is 'POSTER'.
(Other definitions for poster that I've seen before include "Printed advert" , "Bill or placard" , "Paper advertisement for topers" , "Large sheet of paper advertising something" , "Largfe public notice." .)