The most common and most likely answer to this crossword clue is the 8 letter word CREATINE
C
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Buddy explains!
The straight part
We know that "Sports supplement" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sports supplement = creatine.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 CREATINE mean?
CREATINE noun- An amino acid, 2-(carbamimidoyl-methyl- amino)acetic acid, which naturally occurs in vertebrates and helps to supply energy to muscle and nerve cells.