Charm a cross teacher to begin with (6)
I believe the answer is:
amulet
'charm' is the definition.
(amulet is a kind of charm)
'a cross teacher to begin with' is the wordplay.
'cross' becomes 'mule' (I've seen this before**).
'to begin with' suggests taking the first letters.
The initial letter of 'teacher' is 't'.
'a'+'mule'+'t'='AMULET'
(Other definitions for amulet that I've seen before include "Charm protecting against evil" , "Charm, lucky token" , "Small charm" , "Mascot" , "Ornament worn as a charm against evil" .)