Scientist discovered weight in gas (6)
I believe the answer is:
newton
'scientist' is the definition.
(Newton is an example)
'weight in gas' is the wordplay.
'weight' becomes 'wt'.
'in' is an insertion indicator.
'gas' becomes 'neon' (I've seen this before).
'wt' put into 'neon' is 'NEWTON'.
'discovered' acts as a link.
(Other definitions for newton that I've seen before include "He discovered the law of gravity" , "Sir Isaac, discoverer of gravity" , "Great English mathematician who discovered gravity" , "Scientist - unit of force" , "Sir Isaac -, seventeen to eighteenth century English scientist" .)