Extremely noble knight supporting Round Table member? (4)
I believe the answer is:
neon
'table member?' is the definition.
(element on the periodic table)
'extremely noble knight supporting round' is the wordplay.
'extremely' means to remove the middle letters (letters on the extremes of the word are used).
'knight' becomes 'n' (chess abbreviation).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'round' becomes 'o' (round can mean a round object).
'noble' with its middle taken out is 'ne'.
'n' after 'o' is 'on'.
'ne'+'on'='NEON'
(Other definitions for neon that I've seen before include "Gas - light" , "Could it brighten proceedings" , "It may illuminate" , "Sign gas" , "One of the six inert gasses" .)