expression, noun
horse's legs; minor actor playing a horse's legs
Literally 'horse's legs'. Idiomatically refers to a low-ranking actor in traditional theater who plays the rear legs of a stage horse, and by extension any minor or untalented performer relegated to such roles.
彼はまだ駆け出しで、舞台では馬の足をやっている。
He's still a novice and plays the horse's legs on stage.
馬の足役から
He worked his way up from playing the horse's legs and finally landed the lead role.
Compound of 馬 (uma, 'horse') + の (no, possessive particle) + 足 (ashi, 'leg'). The idiomatic meaning comes from traditional Japanese theater, where the rear legs of a stage horse were played by low-ranking actors.