A cross on centre of altar shows charm (6)
I believe the answer is:
amulet
'charm' is the definition.
(amulet is a kind of charm)
'a cross on centre of altar' is the wordplay.
'cross' becomes 'mule' (I've seen this in other clues).
'on' means one lot of letters go next to another.
'centre of' says to take the centre.
The centre of 'altar' is 't'.
'a'+'mule'+'t'='AMULET'
'shows' acts as a link.
(Other definitions for amulet that I've seen before include "Protective trinket" , "Lucky trinket" , "Jewel to protect from witchcraft" , "lucky defender" , "Ornament worn as a charm against evil" .)