Not an ounce lighter (5)
I believe the answer is:
zippo
I believe this clue is a double definition.
'not an ounce' is the first definition.
(both can mean zero or nothing)
'lighter' is the second definition.
(I know that Zippo is a type of lighter)
(Other definitions for zippo that I've seen before include "duck?" , "lighter!" , "Not a bit" .)