Last part is false — authentic originally (4)
I believe the answer is:
coda
'last part' is the definition.
(I've seen this in another clue)
'false authentic originally' is the wordplay.
'false' becomes 'cod' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'originally' indicates taking the first letters.
The initial letter of 'authentic' is 'a'.
'cod'+'a'='CODA'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for coda that I've seen before include "Concluding passage of a musical composition" , "noted conclusion" , "Closing passage of a piece of music" , "Ending of musical piece" , "Final part of mus. structure" .)