The most common and most likely answer to this crossword clue is the 3 letter word HUT
I've seen this crossword clue in The Daily Telegraph
H
U
T
Buddy explains!

The straight part
We know that "Building" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case building = hut.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does HUT mean?
HUT noun- A small, simple one-storey dwelling or shelter, often with just one room, and generally built of readily available local materials.
- A small wooden shed.
- A small stack of grain.
- To provide (someone) with shelter in a hut.
- To take shelter in a hut.
- To stack (sheaves of grain).