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' is an insertion indicator (some letters must 'bore' a hole into other letters).
'philosopher' becomes 'hume' (David Hume).
'endlessly' means to remove the last letter.
'hume' with its last letter removed is 'hum'.
'eli' inserted into 'hum' is 'HELIUM'.
'represents this' is the link.
This may not be correct. Some or all of it may be part of 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" .)