With regard to home, start to replace source of energy (2,2)
I believe the answer is:
in re
'with regard to' is the definition.
(in re means on the topic of)
'home start to replace source of energy' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'start to' suggests taking the first letters.
'source of' suggests taking the first letters.
The first letter of 'replace' is 'r'.
The first letter of 'energy' is 'e'.
'in'+'r'+'e'='IN RE'
(Other definitions for in re that I've seen before include "With regard to" , "Concerning (L)" , "In the matter of (Lat.)" .)