Source of oil in southeast unaltered (6)
I believe the answer is:
sesame
'source of oil' is the definition.
(I've seen this before)
'southeast unaltered' is the wordplay.
'southeast' becomes 'SE' (synonyms).
'unaltered' becomes 'same' (I've seen this in another clue).
'se'+'same'='SESAME'
'in' is the link.
(Other definitions for sesame that I've seen before include "said to be effective in opening spell" , "Opening word for the oilplant?" , "Seeds producing an edible oil" , "Seed or oil used in cooking" , "Password of the Arabian Nights" .)