Tried working with energy and he lost weight (6)
I believe the answer is:
dieter
'he lost weight' is the definition.
The definition suggests a singular noun but the answer is not.
'tried working with energy' is the wordplay.
'working with' indicates anagramming the letters (working with the letters).
'energy' becomes 'e' (physics symbol as in E=mc2).
'tried' with its letters anagrammed with 'e' gives 'DIETER'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for dieter that I've seen before include "fellow who'll eat less?" , "One restricting food" , "German male" , "person who's determined to lose" , "bloke obeying regime" .)