Gas piece from Eugene O'Neill (4)
I believe the answer is:
neon
'gas' is the definition.
(neon is a gas)
'piece from eugene o'neill' is the wordplay.
'piece' indicates a hidden word.
'NEON' is hidden in the letters of 'from eugene oneill'.
(Other definitions for neon that I've seen before include "Inert gas, symbol Ne" , "Type of lamp" , "One of the six inert gasses" , "None (anag.)" , "One of the six inert gases" .)