Nothing I left we’ll openly sink in sources of petroleum gas (3,5)
I believe the answer is:
oil wells
'openly sink in sources of petroleum gas' is the definition.
I know nothing about this answer so I can't judge whether this works.
'nothing i left we'll' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'left' becomes 'l' (common abbreviation).
'we'll' becomes 'wells' (I can't justify this - if you can you should believe this answer much more).
'o'+'i'+'l'+'wells'='OIL-WELLS'
Can you help me to learn more?
(Another definition for oil wells that I've seen is " sources of mineral deposits".)