He might feign ignorance of Post Office's total (6)
I believe the answer is:
possum
'he might feign ignorance' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'post office's total' is the wordplay.
'post office's' becomes 'po's' (abbreviation).
'total' becomes 'sum' (synonyms).
'pos'+'sum'='POSSUM'
'of' is the link.
Can you help me to learn more?
(Other definitions for possum that I've seen before include "Small animal with a pouch" , "American marsupial (that plays dead?)" , "Australian marsupial with dense fur and a long tail" , "The Australasian phalanger" , "Small furry Australian marsupial" .)