One doesn't react, for example, in Eugene O'Neill's piece (5,3)
I believe the answer is:
noble gas
'one doesn't react' is the definition.
(type of gas which isn't very reactive)
'example in eugene o'neill's piece' is the wordplay.
I cannot quite see how this works, but
'example' could be 'as' (I've seen this in other clues) and 'as' is found within the answer.
'piece' could be 'leg' (leg is a kind of piece) and 'leg' is present in the answer.
The remaining letters 'nob' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'for' is the link.
(Other definitions for noble gas that I've seen before include "He symbolises one" , "Such as, eg, radon or neon" , "such as no. 2 in the table" , "Such as helium or neon" , "Periodically it appears on the right" .)