The French sang about energy given by food (7)
I believe the answer is:
lasagne
'food' is the definition.
(lasagne is a kind of food)
'the french sang about energy given' is the wordplay.
'the french' becomes 'la' (**).
'about' is an anagram indicator.
'energy' becomes 'e' (physics symbol as in E=mca).
'given' says to put letters next to each other.
'sang' anagrammed gives 'sagn'.
'la'+'sagn'+'e'='LASAGNE'
'by' acts as a link.
(Other definitions for lasagne that I've seen before include "Eat this as angel" , "The Italian dish is a gas, Len" , "Pasta served in strips" , "Lea sang out for the pasta dish" , "continental dish" .)