Sausage and mash — was butter right substitute for energy? (9)
I believe the answer is:
bratwurst
'sausage' is the definition.
(bratwurst is a kind of sausage)
I don't understand the remainder of the clue.
(Other definitions for bratwurst that I've seen before include "Type of German sausage" , "German pork sausage" , "German port sausage" , "banger" .)