Little weight unit, and another for gas (5)
I believe the answer is:
ozone
'for gas' is the definition.
(I've seen this before)
'little weight unit' is the wordplay.
'little weight' becomes 'oz' (abbreviation for ounce, a unit of weight).
'unit' becomes 'one' (synonyms).
'oz'+'one'='OZONE'
'and another' acts as a link.
(Other definitions for ozone that I've seen before include "Invigorating air of the seaside" , "Sea air; toxic gas" , "Layer that protects against ultraviolet rays" , "Toxic form of oxygen" , "Bracing air (colloq.)" .)