Songs principal heard? (6)
I believe the answer is:
lieder
'songs' is the definition.
(lied is a kind of song)
'principal heard?' is the wordplay.
'principal' becomes 'leader'.
'heard?' indicates a 'sounds like' (homophone) clue.
'leader' sounds like 'LIEDER'.
(Other definitions for lieder that I've seen before include "Relied on German song (6)." , "songs by Schumann?" , "Relied on German song genre" , "Vocal works" , "German art songs" .)