It's a great sacrifice for the old Greeks, as each suspect comes to grave ending perhaps (8)
I believe the answer is:
hecatomb
'it's a great sacrifice' is the definition.
(I've seen this before)
'each suspect comes to grave ending perhaps' is the wordplay.
'suspect' is an anagram indicator (a suspicious or unusual spelling).
'comes to' is a charade indicator (letters next to each other).
'grave ending perhaps' becomes 'tomb' (I can't justify this - if you can you should give a lot more credence to this answer).
'each' anagrammed gives 'heca'.
'heca'+'tomb'='HECATOMB'
'for the old greeks as' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for hecatomb that I've seen before include "great public sacrifice" , "Great quantity of victims" , "mass slaughter" , "sacrifice victims" , "major sacrifice" .)