The most common and most likely answer to this crossword clue is the 8 letter word PROTEINS
I've seen this crossword clue in The Sunday Telegraph
P
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O
T
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S
Buddy explains!

The straight part
We know that "Muscle builders" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case muscle builders = proteins.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 PROTEINS mean?
PROTEINS noun- Any of numerous large, complex naturally-produced molecules composed of one or more long chains of amino acids, in which the amino acid groups are held together by peptide bonds.
- (nutrition) One of three major classes of food or source of food energy (4 kcal/gram) abundant in animal-derived foods (i.e. meat) and some vegetables, such as legumes.
- (nutrition) A food rich in protein, often a meat or meat substitute.