Underworld goddess mostly associated with old doctor’s sacrifice (8)
I believe the answer is:
hecatomb
'sacrifice' is the definition.
(I know that hecatomb is a type of sacrifice)
'underworld goddess mostly associated with old doctor's' is the wordplay.
'underworld goddess' becomes 'hecate' (I've seen this in another clue).
'mostly' means to remove the last letter (most but not all of the word).
'associated with old doctor' becomes 'omb' (I can't explain this - if you can you should believe this answer much more).
'hecate' with its final letter removed is 'hecat'.
'hecat'+'omb'='HECATOMB'
Can you help me to learn more?
(Other definitions for hecatomb that I've seen before include "big sacrifice" , "mass slaughter" , "sacrifice victims" , "major sacrifice" , "Large-scale sacrifice" .)