The most common and most likely answer to this crossword clue is the 4 letter word FATS
I've seen this crossword clue in The Guardian
F
A
T
S
More clues leading to the result FATS
Buddy explains!

The straight part
We know that "Food constituents" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case food constituents = fats.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 FATS mean?
FATS noun- A specialized animal tissue with a high oil content, used for long-term storage of energy.
- A refined substance chemically resembling the oils in animal fat.
- That part of an organization deemed wasteful.
- An erection.
- A poorly played shot where the ball is struck by the top part of the club head. (see also thin, shank, toe)
- The best or richest productions; the best part.
- Work containing much blank, or its equivalent, and therefore profitable to the compositor.
- A fat person.
- A beef cattle fattened for sale.
- A large tub or vessel for water, wine, or other liquids; a cistern.
- A dry measure, generally equal to nine bushels.