Seas seemed worked out as a source of oil (6,4)
I believe the answer is:
sesame seed
'a source of oil' is the definition.
'seas seemed worked out' is the wordplay.
'worked out' is an anagram indicator.
'seas'+'seemed'='seasseemed'
'seasseemed' with letters rearranged gives 'SESAME SEED'.
'as' acts as a link.
(Other definitions for sesame seed that I've seen before include "Oil producer" , "Small oval seeds baked into the crusts of bread or on buns" .)