Crossword Genius

Food constituents employees mostly rejected (4)

Ross

I believe the answer is:

fats

I'm a little stuck... Click here to teach me more about this clue!

'food constituents' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how one could define the other.

'employees mostly rejected' is the wordplay.
'employees' becomes 'staff' (I've seen this before).
'mostly' means to remove the last letter.
'rejected' shows that the letters should be reversed in order.
'staff' with its final letter taken away is 'staf'.
'staf' back-to-front is 'FATS'.

Can you help me to learn more?

(Other definitions for fats that I've seen before include "Eg Waller or Domino" , "Lipids" .)

I've seen this clue in The Guardian.
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