Deity back to scan old letter (5)
I believe the answer is:
thorn
'old letter' is the definition.
(letter of several old european languages)
'deity back to scan' is the wordplay.
'deity' becomes 'thor' (Thor is an example).
'back to' suggests the final letters.
The last letter of 'scan' is 'n'.
'thor'+'n'='THORN'
(Other definitions for thorn that I've seen before include "Sharp-pointed stem-tip" , "Spike on a rose bush" , "Plant spine; old letter" , "Barb" , "sharp object" .)