Oil producer's assets essentially unchanged (6)
I believe the answer is:
sesame
'oil producer's' is the definition.
(producer of sesame oil)
'assets essentially unchanged' is the wordplay.
'essentially' indicates the central letters.
'unchanged' becomes 'same' (similar in meaning).
The centre of 'assets' is 'se'.
'se'+'same'='SESAME'
(Other definitions for sesame that I've seen before include "Open . . . . . . is a magic phrase to gain entry" , "Opening word for the oilplant?" , "'Open - - !' (Ali Baba)" , "Seeds producing an edible oil" , "Source of seed and oil" .)