The most common and most likely answer to this crossword clue is the 9 letter word ADDITIVES
A
D
D
I
T
I
V
E
S
Buddy explains!
The straight part
We know that "supplements" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case supplements = additives.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 ADDITIVES mean?
ADDITIVES noun- A substance added to another substance or product to produce specific properties in the combined substance.
- (grammar) A word or phrase that adds something, such as also, even, or nor.