Breeding exercises understand about energy (8)
I believe the answer is:
pedigree
'breeding' is the definition.
(I've seen this before)
'exercises understand about energy' is the wordplay.
'exercises' becomes 'pe'.
'understand' becomes 'dig' (digging is a kind of understanding).
'about' becomes 're' (regarding**).
'energy' becomes 'e' (physics symbol as in E=mce).
'pe'+'dig'+'re'+'e'='PEDIGREE'
(Other definitions for pedigree that I've seen before include "Line of ancestry, particularly a distinguished one" , "Breeding; descent" , "Genealogical table" , "Line of descent, family tree" , "List of ancestors" .)