Bosom buddy nasty to Samuel (4,4)
I believe the answer is:
soul mate
'bosom buddy' is the definition.
(both can mean a close friend)
'nasty to samuel' is the wordplay.
'nasty' indicates anagramming the letters (a nasty or wrong spelling).
'to'+'samuel'='tosamuel'
'tosamuel' with letters rearranged gives 'SOUL MATE'.
(Other definitions for soul mate that I've seen before include "Friend with a deeply personal relationship, like spiritual spouse" , "One's ideal partner" , "perfect partner" , "One ideally suited to another" , "lover?" .)