Spoil series, revealing the vital part (6)
I believe the answer is:
marrow
'the vital part' is the definition.
(marrow can mean something's core or essence)
'spoil series' is the wordplay.
'spoil' becomes 'mar' (I've seen this before).
'series' becomes 'row' (I've seen this before).
'mar'+'row'='MARROW'
'revealing' is the link.
(Other definitions for marrow that I've seen before include "Matter inside bones, or gourd fruit" , "Soft tissue in bones" , "Stuff found inside bones" , "Vegetable; bone filling" , "Oversized courgette" .)