He represents this priest boring philosopher endlessly (6)
I believe the answer is:
helium
'he' is the definition.
(synonyms)
'priest boring philosopher endlessly' is the wordplay.
'priest' becomes 'eli' (biblical figure).
'boring' means one lot of letters goes inside another (to bore something can mean to drill into it).
'philosopher' becomes 'hume' (David Hume).
'endlessly' means to remove the last letter.
'hume' with its last letter taken away is 'hum'.
'eli' going within 'hum' is 'HELIUM'.
'represents this' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for helium that I've seen before include "Light colourless gas used in balloons" , "Lightest noble gas" , "Floating balloon gas" , "Second lightest element" , "Very light, inert gas" .)