Source of nutrition you nearly left by overturned barrel (4,3)
I believe the answer is:
yolk sac
'source of nutrition' is the definition.
(egg yolks are a source of protein)
'you nearly left by overturned barrel' is the wordplay.
'nearly' means to remove the last letter.
'left by' becomes 'l' (I've seen this in other clues).
'overturned' shows that the letters should be reversed in order.
'barrel' becomes 'cask' (synonyms).
'cask' reversed gives 'ksac'.
'you' with its final letter taken off is 'yo'.
'yo'+'l'+'ksac'='YOLK-SAC'
(Other definitions for yolk sac that I've seen before include "bit of egg" , "Egg membrane" , "Bag containing yellow" , "bag of yellow liquid" , "protection for hens due to hatch?" .)