High seas seemed to be a source of oil (6,4)
I believe the answer is:
sesame seed
'a source of oil' is the definition.
'high seas seemed to be' is the wordplay.
'high' indicates anagramming the letters (like drunk).
'to be' indicates anagramming the letters.
'seas' anagrammed gives 'sesa'.
'seemed' anagrammed gives 'meseed'.
'sesa'+'meseed'='SESAME SEED'
(Other definitions for sesame seed that I've seen before include "Small oval seeds baked into the crusts of bread or on buns" , "Oil producer" .)