Letter in Irish supporting sensationalist preacher (9)
I believe the answer is:
pulpiteer
'preacher' is the definition.
(someone preaching from a pulpit)
'letter in irish supporting sensationalist' is the wordplay.
'letter' becomes 'tee' (the letter T).
'in' indicates putting letters inside.
'irish' becomes 'IR' (abbreviation).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'sensationalist' becomes 'pulp' (as in pulp fiction).
'tee' going inside 'ir' is 'iteer'.
'iteer' put after 'pulp' is 'PULPITEER'.
(Another definition for pulpiteer that I've seen is " Preacher".)