Closest friends crash after consuming Southern punch on retirement (5,4)
I believe the answer is:
bosom pals
'closest friends' is the definition.
'crash after consuming southern punch on retirement' is the wordplay.
'crash' becomes 'boom' (I've seen this before).
'after consuming' means one lot of letters goes inside another.
'southern' becomes 's' (abbreviation).
'punch' becomes 'slap' (both can mean to hit someone).
'on retirement' says the letters should be written in reverse.
'slap' backwards is 'pals'.
'boom' enclosing 's' is 'bosom'.
'bosom'+'pals'='BOSOM PALS'
(Another definition for bosom pals that I've seen is " Very close friends".)