Ruled about case of snake oil provider (7)
I believe the answer is:
linseed
'oil provider' is the definition.
(as in linseed oil)
'ruled about case of snake' is the wordplay.
'ruled' becomes 'lined' (I've seen this in another clue).
'about' indicates putting letters inside.
'case of' suggests removing the centre (like emptying a case).
'snake' with its middle taken out is 'se'.
'lined' placed around 'se' is 'LINSEED'.
(Other definitions for linseed that I've seen before include "Enisled (anag.)" , "Oil applied to cricket bats" , "Flax plant product providing oil" , "Oil-producing plant" , "Oil producer" .)