Tough old bird one keeping house warm (6)

I believe the answer is:

boiler

I'm a little stuck... Click here to teach me more about this clue!

'tough old bird' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for boiler that I've seen before include "Container to heat water or tough fowl" , "Water heater" , "Water-heater" , "Steam-producer" , "Container in which water is heated" .)

I've seen this clue in The Sun.

Want a hint initially instead of a full solution? Install my app