A cross that's primarily a lucky charm (6)
I believe the answer is:
amulet
'a lucky charm' is the definition.
(I've seen this before)
'a cross that's primarily' is the wordplay.
'cross' becomes 'mule' (I've seen this before**).
'primarily' suggests taking the first letters.
The first letter of 'that' is 't'.
'a'+'mule'+'t'='AMULET'
(Other definitions for amulet that I've seen before include "Mascot" , "Trinket worn to protect one from evil" , "Protective jewellery" , "Piece of jewelry thought to be a protection against evil" , "Charm worn against bad luck" .)