He's seeing Pong entertaining priest (6)
I believe the answer is:
helium
'he's' is the definition.
('helium' can be a synonym of 'he')
'pong entertaining priest' is the wordplay.
'pong' becomes 'hum' (I've seen this before).
'entertaining' means one lot of letters goes inside another.
'priest' becomes 'eli' (I've seen this before).
'hum' going around 'eli' is 'HELIUM'.
'seeing' is the link.
(Other definitions for helium that I've seen before include "... ditto: symbol He" , "Lightest noble gas" , "Floating balloon gas" , "Very light, inert gas" , "Light colourless gas, sometimes in balloons" .)