This could produce oil from river and deep sea, possibly
I believe the answer is:
rapeseed
'this could produce oil' is the definition.
'river and deep sea possibly' is the wordplay.
'river' becomes 'r'.
'and' says to put letters next to each other.
'possibly' indicates anagramming the letters.
'deep'+'sea'='deepsea'
'deepsea' anagrammed gives 'apeseed'.
'r'+'apeseed'='RAPESEED'
'from' acts as a link.
(Other definitions for rapeseed that I've seen before include "Oil producer" , "Part of plant yielding oil" , "Oil-rich crop" , "Source of cooking oil" .)