Attachment from down under going astray in the mail, PS left out (8)
I believe the answer is:
mateship
'attachment from down under' is the definition.
(Australian term for being friends or mates)
'going astray in the mail ps left out' is the wordplay.
'going astray in' indicates an anagram.
'left' becomes 'L' (common abbreviation).
'out' suggests deleting specific letters.
'the'+'mail'+'ps'='themailps'
'themailps' with 'l' removed is 'themaips'.
'themaips' is an anagram of 'MATESHIP'.
(Other definitions for mateship that I've seen before include "amity in Perth" , "bond between close friends" , "Aussie bond between friends" .)