Steam generator (6)
I believe the answer is:
boiler
'steam generator' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for boiler that I've seen before include "Container to heat water or tough fowl" , "Tank for heating water" , "Container for heating water or a tough fowl" , "Water heater" , "tough old bird" .)