Name one, generating gas (4)
I believe the answer is:
neon
'gas' is the definition.
(neon is a gas)
'name one generating' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'generating' is an anagram indicator.
'one' anagrammed gives 'eon'.
'n'+'eon'='NEON'
(Other definitions for neon that I've seen before include "Colourless noble gas" , "Type of lamp" , "Sign gas" , "Inert gas, symbol Ne" , "Type of lighting" .)