Measure of force not weight in a gas (4)
I believe the answer is:
neon
'gas' is the definition.
(neon is a gas)
'measure of force not weight' is the wordplay.
I cannot really understand how this works, but
'measure' could be 'eon' (eon is a kind of measure) and 'eon' is present in the answer.
A single letter 'n' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'in a' acts as a link.
Can you help me to learn more?
(Other definitions for neon that I've seen before include "Brightly coloured advertising sign" , "Type of lamp" , "Element 10, used in lights" , "Colourless noble gas" , "One of the six inert gases" .)