A piece of advice by the French teacher (7)
I believe the answer is:
apostle
'teacher' is the definition.
(I have seen 'Christian teacher' mean 'apostle' so perhaps 'teacher' could also mean 'apostle')
'a piece of advice by the french' is the wordplay.
'piece of advice' becomes 'post' (I am not sure about this - if you are sure you should believe this answer much more).
'by' means one lot of letters go next to another.
'the french' becomes 'le' ('the' in French).
'a'+'post'+'le'='APOSTLE'
Can you help me to learn more?
(Other definitions for apostle that I've seen before include "Early Christian missionary" , "Missionary or evangelist" , "[JAMES] , for one?" , "Advocate" , "eg John, Paul" .)