The most common and most likely answer to this crossword clue is the 9 letter word COLOSTRUM
I've seen this crossword clue in The Times
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The straight part
We know that "a nutritious substance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a nutritious substance = colostrum.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does COLOSTRUM mean?
COLOSTRUM noun- A form of milk produced by the mammary glands in late pregnancy and the few days after giving birth. Human and bovine colostrum is thick and yellowish. In humans, it has high concentrations of nutrients and antibodies, but it is small in quantity.
- A mixture of turpentine and egg yolk, formerly used as an emulsion.