The game of sorting odd mail (3,4)
I believe the answer is:
old maid
'the game' is the definition.
(I've seen this before)
'sorting odd mail' is the wordplay.
'sorting' indicates anagramming the letters.
'odd'+'mail'='oddmail'
'oddmail' with letters rearranged gives 'OLD MAID'.
'of' acts as a link.
(Other definitions for old maid that I've seen before include "Elderly single woman - card game" , "Prim type; card game" , "Pejorative term for elderly unmarried woman" , "Obsolete expression for elderly unmarried woman" , "She's elderly and unmarried" .)