Sing about high priest's balloon content? (6)
I believe the answer is:
helium
'balloon content?' is the definition.
(helium is often used to fill balloons)
'sing about high priest's' is the wordplay.
'sing' becomes 'hum' (humming is a kind of singing).
'about' means one lot of letters goes inside another.
'high priest' becomes 'eli' (biblical figure).
'hum' placed around 'eli' is 'HELIUM'.
(Other definitions for helium that I've seen before include "Lightest noble gas" , "Balloon-filling element" , "Light colourless gas used in airships" , "Second lightest element" , "Floating balloon gas" .)