Use of fret in employing energy (6)
I believe the answer is:
effort
'energy' is the definition.
(thesaurus)
'use of fret' is the wordplay.
'use' indicates anagramming the letters.
'of'+'fret'='offret'
'offret' is an anagram of 'EFFORT'.
'in employing' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for effort that I've seen before include "Toil" , "The use of physical or mental energy" , "Exertion - attempt - achievement" , "A bid or endeavour" , "Endeavour, elbow grease" .)