Roobarb originally a French character (4)
I believe the answer is:
rune
'character' is the definition.
(rune is a kind of character)
'roobarb originally a french' is the wordplay.
'originally' indicates taking the first letters.
'a french' becomes 'une' ('a' in French).
The first letter of 'roobarb' is 'r'.
'r'+'une'='RUNE'
(Other definitions for rune that I've seen before include "Spell" , "Letter of an ancient Germanic alphabet" , "Ancient symbol" , "Old Scandinavian letter" , "Ancient character" .)