Bring up barrel left with vacuous orderly, that contains nutritious yellow substance (4,3)
I believe the answer is:
yolk sac
'nutritious yellow substance' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'bring up barrel left with vacuous orderly' is the wordplay.
'bring up' says the letters should be written in reverse.
'barrel' becomes 'cask' (synonyms).
'left with vacuous orderly' becomes 'loy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cask'+'loy'='caskloy'
'caskloy' written backwards gives 'YOLK-SAC'.
'that contains' is the link.
Can you help me to learn more?
(Other definitions for yolk sac that I've seen before include "bag of yellow liquid" , "original food parcel" , "small food parcel" , "Something feeding an embryo" , "Bag containing yellow" .)