Oils generated in the earth (4)
I believe the answer is:
soil
'the earth' is the definition.
(earth can mean soil or dirt)
'oils generated' is the wordplay.
'generated' indicates an anagram.
'oils' is an anagram of 'SOIL'.
'in' acts as a link.
(Other definitions for soil that I've seen before include "In which plants grow" , "Dirty - dirt" , "Loose upper layer of earth" , "Sully with earth" , "Earth; make dirty" .)