Unit of force giving appearance of weight in light gas (6)
I believe the answer is:
newton
'unit of force' is the definition.
(I've seen this before)
'weight in light gas' is the wordplay.
'weight' becomes 'wt'.
'in' indicates putting letters inside.
'light gas' becomes 'neon'.
'wt' inserted into 'neon' is 'NEWTON'.
'giving appearance of' is the link.
(Other definitions for newton that I've seen before include "Isaac . . . . . ., great English physicist who discovered gravity" , "Explainer of gravity" , "Famous mathematician" , "He discovered the law of gravity" , "Great English mathematician who discovered gravity" .)