After a change of heart, rabbit in French language (5)
I believe the answer is:
latin
'french language' is the definition.
'latin' can be an answer for 'language' (Latin is an example). I'm not sure about the 'french' bit.
'after a change of heart rabbit in' is the wordplay.
'after' means one lot of letters go next to another.
'a change of heart rabbit' becomes 'lat' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'lat' next to 'in' is 'LATIN'.
Can you help me to learn more?
(Other definitions for latin that I've seen before include "Parent language of French,Romanian" , "Language of the Caesars" , "Cicero's tongue" , "language class" , "Learning it 'an honour' (Churchill)" .)