Find fault in friendly source of power (4,4)
I believe the answer is:
wind farm
'source of power' is the definition.
(energy source)
'find fault in friendly' is the wordplay.
'fault' indicates anagramming the letters (a faulty spelling).
'in' means one lot of letters goes inside another.
'friendly' becomes 'warm' (synonyms).
'find' is an anagram of 'indf'.
'indf' placed into 'warm' is 'WIND FARM'.
(Other definitions for wind farm that I've seen before include "Area of power turbines" , "generator" , "Group of aerial rotors generating electricity" , "energy producer" , "Alternative plant for generating electricity" .)