Old money with capitalist’s margins gone forever (7)
I believe the answer is:
extinct
'gone forever' is the definition.
(I've seen this before)
'old money with capitalist's margins' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'money' becomes 'tin' (old-fashioned slang term for money).
'with' is a charade indicator (letters next to each other).
'margins' means to remove the middle letters (the letters on the margins are used).
'capitalist' with its middle removed is 'ct'.
'ex'+'tin'+'ct'='EXTINCT'
(Other definitions for extinct that I've seen before include "(Creature) no longer about" , "No more vital" , "Having died out, like the dinosaur" , "Dead - obsolete - extinguished" , "Of volcano, no longer active" .)