The most common and most likely answer to this crossword clue is the 5 plus 7 letter word EARLY ENGLISH
I've seen this crossword clue in The Daily Telegraph
E
A
R
L
Y
E
N
G
L
I
S
H
Buddy explains!
The straight part
We know that "Old style of building" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old style of building = early english.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.