One who has deserted a post office, say (8)
I believe the answer is:
apostate
'one who has deserted' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a post office say' is the wordplay.
'post office' becomes 'po' (abbreviation).
'say' becomes 'state' (synonyms).
'a'+'po'+'state'='APOSTATE'
Can you help me to learn more?
(Other definitions for apostate that I've seen before include "He renounces former religion - may opt at sea" , "heretic" , "Dissenter" , "Someone who has abandoned their religion" , "Religious rebel" .)