Principal supplier of gas or water (4)
I believe the answer is:
main
'water' is the definition.
(I know that main is a type of water)
'principal supplier of gas' is the wordplay.
'principal supplier of' becomes 'mai' (I can't justify this - if you can you should give a lot more credence to this answer).
'gas' becomes 'N' (chemical symbol for nitrogen).
'mai'+'n'='MAIN'
'or' is the link.
Can you help me to learn more?
(Other definitions for main that I've seen before include "Source of electricity" , "source of gas" , "Most important element" , "river in Germany" , "Principal sea" .)