When describing a person as being "lean-fleshed" there are a few possible synonyms that could be used to convey the same idea. Some alternative words might include "slender," "thin," "athletic," or "toned." Each of these could be used to describe someone who appears to have a lower body fat percentage, and may have well-defined musculature as a result. Depending on the context, other synonyms that might be considered could include "fit," "svelte," "willowy," or "lithe." Ultimately, the most appropriate synonym would depend on the specific circumstances, tone, and intended meaning of the phrase in question.