Tout upset priest about ticket, ultimately (7)
I believe the answer is:
tipster
'tout' is the definition.
('tipster' can be a synonym of 'tout')
'upset priest about ticket ultimately' is the wordplay.
'upset' indicates an anagram.
'about' means one lot of letters goes inside another.
'ultimately' suggests the final letters.
The last letter of 'ticket' is 't'.
'priest' is an anagram of 'tipser'.
'tipser' enclosing 't' is 'TIPSTER'.
(Other definitions for tipster that I've seen before include "he recommends risky investments" , "Gambling advisor" , "Racing tout" , "Race predictor" , "One advising betters" .)