Convert finally changed priest for one offering better advice (7)
I believe the answer is:
tipster
'one offering better advice' is the definition.
(someone giving betting tips)
'convert finally changed priest' is the wordplay.
'finally' indicates one should take the final letters.
'changed' indicates an anagram.
The last letter of 'convert' is 't'.
'priest' anagrammed gives 'ipster'.
't'+'ipster'='TIPSTER'
'for' is the link.
(Other definitions for tipster that I've seen before include "One predicting winners" , "One with views on race" , "Adviser on betting" , "Racing tout" , "Race predictor" .)