Get upset about unknown people no longer with us (4)
I believe the answer is:
exes
'people no longer with us' is the definition.
(as in ex-partners)
'get upset about unknown' is the wordplay.
'get' becomes 'see' (I've seen this in other clues).
'upset' says the letters should be written in reverse.
'about' indicates putting letters inside.
'unknown' becomes 'x' (algebra).
'see' back-to-front is 'ees'.
'ees' placed around 'x' is 'EXES'.
(Other definitions for exes that I've seen before include "Spouses once" , "Reduced claim from employer" , "old dates" , "Claims made" , "former union members" .)