Measures for the energy value of food (8)
I believe the answer is:
calories
'measures for the energy value of food' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for calories that I've seen before include "Measures for the energy value of food" , "Dieters count them" , "Units of energy in food" , "Energy or heat units from Escorial" , "Measures for the energy potential of food" .)