Bit of egg only spilt after throwing new vessel up (4,3)
I believe the answer is:
yolk sac
'bit of egg' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for yolk sac that I've seen before include "Something feeding an embryo" , "Bag containing yellow" , "soldiers may go for a dip" , "Provider of sustenance to an embryo" , "bag of yellow liquid" .)