Leaving group after ultimately futile sacrifice (8)
I believe the answer is:
offering
'sacrifice' is the definition.
(eg an offering to a god)
'leaving group after ultimately futile' is the wordplay.
I cannot really see how this works, but
'leaving' could be 'off' (as in 'I'm off') and 'off' is located in the answer.
'group' could be 'ring' (a ring is a criminal organisation) and 'ring' is found within the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for offering that I've seen before include "Gift presented for acceptance" , "Contribution, sacrifice" , "Present" , "making a bid" , "Putting forward for acceptance or rejection" .)