Ruled without second energy source of oil (7)
I believe the answer is:
linseed
'source of oil' is the definition.
(I've seen this before)
'ruled without second energy' is the wordplay.
'ruled' becomes 'lined' (I've seen this before).
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'second' becomes 's' (common abbreviation).
'energy' becomes 'e' (physics symbol as in E=mc2).
's'+'e'='se'
'lined' enclosing 'se' is 'LINSEED'.
(Other definitions for linseed that I've seen before include "Treatment for bats" , "Enisled (anag.)" , "Flax product, a source of oil" , "Oil applied to cricket bats" , "Oil produced from flax" .)