The French money regularly used by Caesar (5)
I believe the answer is:
latin
'regularly used by caesar' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'the french money' is the wordplay.
'the french' becomes 'la' (one of the French words for 'the').
'money' becomes 'tin' (old-fashioned slang term for money).
'la'+'tin'='LATIN'
Can you help me to learn more?
(Other definitions for latin that I've seen before include "demanding subject" , "Shakespeare's was small, according to Jonson" , "The language of old Rome" , "Ancestor of French, Romanian" , "Classical language" .)