Building very warm reduced energy bill finally (5)
I believe the answer is:
hotel
'building' is the definition.
(hotel is a kind of building)
'very warm reduced energy bill finally' is the wordplay.
'very warm' becomes 'hot' (I've seen this before).
'reduced' says to take the initial letters (the word is reduced to its first letter).
'finally' says to take the final letters.
The first letter of 'energy' is 'e'.
The last letter of 'bill' is 'l'.
'hot'+'e'+'l'='HOTEL'
(Other definitions for hotel that I've seen before include "Overnight accommodation" , "Fawlty Towers, say" , "Lodging house - code for "H"" , "Oh, let it take guests" , "Accommodating establishment" .)