Sacking second-class subeditor upset newcomer (8)
I believe the answer is:
outsider
'newcomer' is the definition.
(thesaurus)
'sacking second-class subeditor upset' is the wordplay.
'sacking' indicates named letters should be taken away.
'second class' becomes 'b'.
'upset' indicates anagramming the letters.
'subeditor' with 'b' taken away is 'sueditor'.
'sueditor' anagrammed gives 'OUTSIDER'.
(Other definitions for outsider that I've seen before include "Not one of the in-crowd" , "expected to be unsuccessful" , "Foreigner" , "Stranger" , "It's not among the favourites to bet on" .)