In an undertone (5,4)
I believe the answer is:
sotto voce
'in an undertone' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sotto voce that I've seen before include "(Said) quietly" , "In an undertone from Italy" , "Spoken quietly" , "on the quiet" , "In an undertone, from Italian" .)