Singer seen in - 'The Waltons (4)
I believe the answer is:
alto
'singer' is the definition.
(type of singing voice)
'in 'the waltons' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALTO' is hidden within 'waltons'.
'seen' is the link.
(Other definitions for alto that I've seen before include "Voice part" , "One who sings" , "Singing voice between tenor and soprano" , "Key old > male who scores high" , "Woman's singing voice" .)