Talisman, a cross, original in truth (6)
I believe the answer is:
amulet
'talisman' is the definition.
(synonyms)
'a cross original in truth' is the wordplay.
'cross' becomes 'mule' (a mule is a cross of a donkey and a horse).
'original in truth' becomes 't' (I can't justify this - if you can you should believe this answer much more).
'a'+'mule'+'t'='AMULET'
Can you help me to learn more?
(Other definitions for amulet that I've seen before include "A type of lucky charm" , "Ornament worn as a charm against evil" , "lucky defender" , "Charm worn to ward off evil" , "Charm, lucky token" .)