Hello' sung falsetto? (4)
I believe the answer is:
high
'falsetto?' is the definition.
('high' can be similar in meaning to 'falsetto')
'hello' sung' is the wordplay.
'hello'' becomes 'hi' (synonyms).
'sung' shows a homophone (sound like) (if it is sung it is spoken as a sound).
'hi' sounds like 'HIGH'.
(Other definitions for high that I've seen before include "Shrill" , "Drunk; anticyclone" , "Elevated; euphoric" , "Important, elevated" , "Important; drugged" .)