54 year old historian (4)
I believe the answer is:
livy
'old historian' is the definition.
'Livy' can be an answer for 'historian' (Roman historian). I'm not certain of the 'old' bit.
'54 year' is the wordplay.
'54' becomes 'liv' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'year' becomes 'y'.
'liv'+'y'='LIVY'
Can you help me to learn more?
(Other definitions for livy that I've seen before include "Poet" , "Roman historian" .)