Diet of cold meat and ale — less energy (7)
I believe the answer is:
chamber
'diet' is the definition.
(thesaurus)
'cold meat and ale less energy' is the wordplay.
'cold meat' becomes 'cham' (I can't justify this - if you can you should give a lot more credence to this answer).
'and' means one lot of letters go next to another.
'ale' becomes 'beer' (ale is a type of beer).
'less' indicates named letters should be taken away (less can mean minus).
'energy' becomes 'e' (physics symbol as in E=mch).
'beer' with 'e' taken away is 'ber'.
'cham'+'ber'='CHAMBER'
'of' is the link.
Can you help me to learn more?
(Other definitions for chamber that I've seen before include "posh room" , "boudoir?" , "A house of parliament" , "Room where a judge transacts business" , "Official room" .)