Lightweight unit of gas (5)
I believe the answer is:
ozone
'gas' is the definition.
(I've seen this before)
'lightweight unit' is the wordplay.
'lightweight' becomes 'oz' (abbreviation for ounce, a light weight).
'unit' becomes 'one' (synonyms).
'oz'+'one'='OZONE'
'of' acts as a link.
(Other definitions for ozone that I've seen before include "something found in the atmosphere" , "Atmospheric gas shielding us from ultraviolet radiation" , "Invigorating air of the seaside" , "By sea, you could take in this" , "Ring" .)