Mathematical function describing energy in river (5)
I believe the answer is:
seine
'river' is the definition.
(river in France)
'mathematical function describing energy' is the wordplay.
'mathematical function' becomes 'sine' (sine is a kind of mathematical function).
'describing' is an insertion indicator (describe can mean to form the outline of).
'energy' becomes 'e' (physics symbol as in E=mc2).
'sine' enclosing 'e' is 'SEINE'.
'in' acts as a link.
(Other definitions for seine that I've seen before include "Parisian river" , "Second longest river in France" , "Paris river" , "French river -- type of fishing net" , "Paris's river" .)