Quantity of energy given by a component of salad? (6)
I believe the answer is:
pepper
'component of salad?' is the definition.
(a salad might contain peppers)
'quantity of energy given by a' is the wordplay.
'quantity of energy given' becomes 'pep' (I can't justify this - if you can you should believe this answer much more).
'by' says to put letters next to each other.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'pep'+'per'='PEPPER'
(Other definitions for pepper that I've seen before include "Spicy condiment" , "A hot condiment, may be red" , "... which can make one sneeze ..." , "Riddle" , "Pungent seasoning and vegetable" .)