Gas that could make Noel rich (8)
I believe the answer is:
chlorine
'gas' is the definition.
(I know that chlorine is a type of gas)
'could make noel rich' is the wordplay.
'could make' is an anagram indicator.
'noel'+'rich'='noelrich'
'noelrich' with letters rearranged gives 'CHLORINE'.
'that' acts as a link.
(Other definitions for chlorine that I've seen before include "element - that's dangerous!" , "Toxic green gas" , "Gas used in STERILISING water and in industry (8)" , "Poison gas, with sodium makes salt" , "Halogen gas, Cl" .)