The Big Bang ends in an ellipsoid (3)
I believe the answer is:
egg
'an ellipsoid' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'the big bang ends' is the wordplay.
'ends' suggests the final letters.
The final letters of 'the big bang' are 'EGG'.
'in' is the link.
Can you help me to learn more?
(Other definitions for egg that I've seen before include "It, or chicken, first?" , "Over-easy breakfast item?" , "Baby carrier?" , "It's laid by hen" , "explosive" .)