Tout upset priest after Towcester's first (7)
I believe the answer is:
tipster
'tout' is the definition.
('tipster' can be a synonym of 'tout')
'upset priest after towcester's first' is the wordplay.
'upset' indicates an anagram.
'after' says to put letters next to each other.
'first' says to take the initial letters.
The initial letter of 'towcester' is 't'.
'priest' with letters rearranged gives 'ipster'.
'ipster' put after 't' is 'TIPSTER'.
(Other definitions for tipster that I've seen before include "better counsellor" , "Supposed expert on horses" , "Race predictor" , "he recommends risky investments" , "Seller of information on racecourse etc" .)