Charm, a cross, first of three (6)
I believe the answer is:
amulet
'charm' is the definition.
(amulet is a kind of charm)
'a cross first of three' is the wordplay.
'cross' becomes 'mule' (I've seen this before).
'first of three' becomes 't' (1st letter of 'three').
'a'+'mule'+'t'='AMULET'
(Other definitions for amulet that I've seen before include "Charm protecting against evil" , "Lucky ornament" , "Charm to protect from evil" , "Charm worn to ward off evil" , "Thing worn as charm against evil" .)