Greek character discovering some gas (5)
I believe the answer is:
omega
'greek character' is the definition.
(letter from the Greek alphabet)
'discovering some gas' is the wordplay.
'discovering' says the answer is hidden in the clue (I've seen 'discovered' mean this (remove the letters covering the answer in the clue)).
'OMEGA' can be found hidden inside 'some gas'.
(Other definitions for omega that I've seen before include "Ultimate letter" , "symbol of resistance?" , "Conclusion" , "Final outcome" , "end of series" .)