undersong

noun3

1. •
   An accompanying sound or strain; an accompaniment.

   • Accept the Off'ring I at distance bring, / With Harp ill-tun'd, and long thro' Age unstrung, / Fit only to fill up some Under-song! — 1702
2. •
   (figurative) Subordinate and underlying idea, meaning or atmosphere; undertone.

   • The mad smouldering lust which gives a sort of under-song of surging passion to the sophisticated sensuality of “Salome” … — 1916
3. •
   (no longer used) The burden of a song; the chorus; the refrain.

   • The Challenge to Damaetas shall belong / Menalcas shall sustain his under Song: / Each in his turn your tuneful numbers bring; / In turns the tuneful Muses love to sing. — 1697