פירוש על במדבר 4:47

עבדת עבדה THE SERVICE OF THE MINISTRY — (lit., a service of a service). This is the song of the Levites with accompaniment of cymbals and harps, this being the service attached to another service (the sacrificial service) (Arakhin 11a).

Service for [another] service. Meaning that at the time when the Kohanim were making offerings and dealing with the service, the Levites were singing and playing on their cymbals and harps. Thus they performed a service for the service of the Kohanim.