Doctors, in sum, given to single marker of death (9)
I believe the answer is:
tombstone
'marker of death' is the definition.
(a tombstone marks a grave)
'doctors in sum given to single' is the wordplay.
'doctors' becomes 'MBs' (Bachelors of Medicine).
'in' indicates putting letters inside.
'sum' becomes 'tot' (synonyms).
'given to' means one lot of letters go next to another.
'single' becomes 'one' (similar in meaning).
'mbs' put inside 'tot' is 'tombst'.
'tombst'+'one'='TOMBSTONE'
(Other definitions for tombstone that I've seen before include "Last monument" , "Memorial in churchyard - town in Arizona" , "Graveyard marker" , "Does it display departure date" , "film location" .)